I had a 2006 Intel iMac that died (logic board / graphics card failure) and instead of consigning it to the bin, I aimed to turn it both an external monitor for my screen-less 2009 MacBook Pro (LCD driver failure, failing battery) and any HDMI device I wanted. I had seen this done with G5 iMacs, but never with Intel ones, which use different display types, so decided to have a go!
Step 1: Gut the IMac
Firstly, I completely gutted the iMac, safely keeping the LCD screen to one side. I also repaired any torn EMI shielding and shielded all holes, with the exception of the power in, vent above this. (I also left the 3x USB ports free to fit a USB hub, but this didn't work great, so ended up being removed).
You might be able to make a bit of money selling the various removed parts on eBay.
Step 2: Find a LCD Display Driver Board
I next found out that my screen was a Samsung LTM201M1-L01 and after scouring eBay with this model number, I managed to find a display driver for this screen for £30 inc delivery from the seller e-qstore - http://stores.ebay.co.uk/eqstore/
Your mileage may vary depending on whether your iMac has this screen or another.
My LCD driver board has VGA, DVI and HDMI in and is powered by a 12V 4A external PSU - the eBay seller also sold PSUs, but I had one kicking about from another LCD screen.
Step 3: Test Driver Board
Before assembling it all, it is worth testing the driver board works fine - I had some issues with the Mini Display port to HDMI cable I bought, so returned this and bought a 1M Mini Display port to DVI cable instead which worked fine. I also bought a 1M white HDMI cable to have as an extra input cable for everything else and a 0.5 Metre 2.1mm DC Barrel Extension Cable, to allow me to connect and disconnect the PSU from outside the case.
Step 4: Assemble! - Part 1
The various mounting posts inside the iMac were not suitable, so I purchased some PCB self-adhesive mounting posts (9.5mm) to mount the driver board, inverter and controller board. To ensure the mounting posts were mounted correctly, I cut squares out of the EMI shielding before attaching the posts to the inner of the iMac casing. I also drilled holes for the power / adjustment board, so the buttons could be accessed from the rear of the machine.
Step 5: Assemble! - Part 2
From here it was a just a matter of attaching the boards to the PCB posts, hooking up the
various cables and feeding through them through the former power in cut-out. I also insulated under the LCD driver board, as it was a dear circuit board. The inverter came contained within a clear plastic case. As with the original setup, the LVDS cable and bottom inverter cables could only be attached with the screen in place.
Step 6: Complete!
I then reassembled the iMac, attaching the screen, repairing EMI tape then putting the front on! It now works perfectly with the Mini DisplayPort output of my screen-less MacBook Pro and I have an available HDMI cable to attach any other device. The only thing that was not changeable was the CCFL brightness, but using Shades preference pane solves this satisfactorily for me.
Step 7: Bonus Stage - Rear Mount MacBook
To keep everything neat, and as I had a separate keyboard and Magic Mouse, I also wanted to rear mount the Macbook - I created a mount from wood in a similar style to the AluRack, mounting onto the cable hole on the iMac stand - perfect!