This project is to use an old PlayStation console as a shell for a Raspberry Pi.

If you don't know how to solder or are not able to safely handle electricity, please don't do this project.

The goal is for all of the ports to be in the back and to be able to use PlayStation controllers through the original front ports. A lot of this project involves careful application of hot glue and plastic-cutting/grinding and I'm not going to describe every step (as this involves hours of careful work).

Here are some parts you will need for this project:

1x 2amp 5v power supply with the micro USB end to power the Raspberry Pi.
1x Raspbery Pi 2
1x 2-port PlayStation controller USB adapter (that has been confirmed to work with RPI)
2x USB short extension cable (male/female) with Right/Left angle
1x HDMI short extension cable (male/female)
1x Ethernet short extension cable (male/female)
1x USB 2.5-inch hard-drive with enclosure/wire
1x Set of Heat Sinks for Raspberry Pi (optional, but recommended)

And some tools:

Hot Glue Gun
Super Glue
Screwdriver with Phillips bits
Soldering Iron
Multi-Meter (or continuity tester)
Dremel or Rotary tool with various bits
Pliers (needle-nosed preferably)
Fine/Small flat-file (1/4 inch wide)
Picks (optional)
Wire Strippers
Wire cutters (for wire and for plastic)
A sharp carving knife
Your wits.

Step 1: Disassemble the PlayStation and Prepare Power Board

This and the next step can be done out of order. As I was figuring this out as I went, I did both at the same time (so pics do reflect that a bit).

Remove everything from the PlayStation shell, first.
Then take the power board and remove all components (caps, resistors, etc) except for the power jack, the two switches for power and reset, the LED, and the fuse holder. For this project, I don't use the reset switch, but it's just left there so that the button will mechanically function.
Take a dremel and go around the power and reset switches to isolate them from the circuit.
Wire a circuit from the jack in the back, using the fuse, through the power switch (so that the circuit can be completed only when the power switch is on). Use a decent gauge wire (as shown) for this to ensure the wire doesn't overheat.
Run the positive/negative wires from the power switch up to the top-side of the board.
On your USB power cable, remove the housing and wall-socket prongs (leaving only the board inside).
Now wire the positive/negative cables from the PlayStation power switch to the cable power board.
Taking note of the polarity (LEDs are diodes), desolder the power LED on the USB power cable board and wire the LED positive/negative points to the PlayStation LED using the same polarity.
Now, using the plastic you cut off and some hot clue, glue the cable power board down to the PlayStation board - being careful to isolate the board from any metal sticking up through the board (I used the plastic as a layer between the boards).
If you have too much cable, just coil it up and leave it somewhere in the PlayStation (I glued mine down to the power board (as shown). Make sure the top fits and if you need to, dremel a gap to fit the wire from the left cavity to the center cavity.
Check everything for continuity and voltage (ensure 5v is coming out of USB end only when switched on and that LED works).

Step 2: Prepare Controller Ports With USB Adapter

Unscrew the middle portion where the controller ports are.
Remove and discard the metal shielding.
Remove the inside of the controller connector in the housing - leaving only a hole for the male end of the controller to go into.
The USB adapter cable will have a female PlayStation end. This end will have a D-shaped end for a PlayStation controller to fit in. Remove the curved portion of this D-Shaped end and leave the flat part with the metal connector in the center intact.
Fit the two together, thinning the flat-shaped adapter part until it sits flush in the housing and allows a controller to connect snugly.
Use super glue to glue the flat plastic portion to the inside of the housing. While the glue is setting up, it might be good to use a controller to hold everything together and ensure it's going to fit later.
Use hot glue to secure the rest of the adapter to the housing - taking care that you will be able to screw the housing back into the PlayStation.
Secure left-over cable with hot glue (as shown later).

Step 3: Add Hard-Drive, Raspberry Pi, and Close Case

Add in your hard-drive and Raspberry Pi and connect the USB cables.
Remove any case plastic that gets in your way, but be sure to leave the 6 plastic poles for screws to go into.
The two L/R USB angled cables should go in the two bottom USB ports on the Raspberry Pi
The top two USB ports are used for the drive and controller adapter.
Use Ethernet and HDMI extension cables as well (I had to cut/grind away from the cable ends and the case, etc to get these to fit in place in the shell, so don't be afraid to do that).
Before closing the shell, dremel out the ports in the back where the cable ends can fit in and secure them with hot glue. The ports will split in half from top/bottom halves of the shell so be careful to keep symmetry while cutting/filing.

I ended up gluing the two USB ends into the plastic piece that snaps in (then gluing that piece to the bottom - leaving the top free to slide into the top), the Ethernet port to the bottom half, and the HDMI port to the upper half (because it had to bend backwards and avoid the GPIO pins on the Raspberry Pi). When closing the halves, everything should fit snugly, but don't force it. Find out what's blocking it from closing and address by shifting cables and/or securing them with hot glue. You don't want to crush anything or bend anything here.

I use a pick to move cables around from the top hole. You don't want any cable touching the Pi where it's going to get hot.

Step 4: Set Up Your Raspberry Pi Software.

Not really going to go into this as there are plenty of guides for this (I used RetroPie for mine).

There's one thing worth noting. While the USB ports on the Pi are powered, they're probably not getting enough current to run the external drive (meaning the drive won't spin up or mount). So to solve this and ensure the drive will mount, you'll want to direct more current to the USB. This is done via software, doubling the current over the USB bus from 600mA to 1200mA (affecting the GPIO I believe - which we're not using). Please see this guide for that: http://www.htpcguides.com/power-2-5-hard-drive-with-raspberry-pi-b/