Ok, this is the last step I know of to repair an Xbox with RROD. It's also the most difficult and costly. Be aware of this before you begin!
Recall back to the beginning of this tuturiol when I described how chips now are using little balls of solder in place of contact pins (BGA afterall stands for Ball Grid Array). If you've ever soldered, you know that generally, there is a limit to how often and how much heat you can put solder through before it gets 'gunky' and doesn't flow, and no longer has that beautiful silver glint, but rather a pale, greyish coating. In these events, you generally replace the bad solder with new/good solder. Reballing is exactly that.
The process, in a nutshell, involves actually removing the chip from the board (gotta get it back up to 220+ degrees celsius again, and this time pull the chip off while solder is liquid). Once removed, you can use a combination of flux (to distribute heat more effectively), solder braid (absorbs melted solder in a ribbon of braided copper wires), and your soldering iron/pen to completely clean both surfaces (processor and board). Once clean, you apply a stencil with a bunch of holes to the chip (usually with bracket or rig of some sort), and then you can scoop almost microscopic beads of solder (might I suggest going with the LEADED solder this time, since the problems are most likely fault of lead-free solder?) into the holes in the stencil, heat the solder back to melting temp (around 220 again), and once set, remove the stencil leaving the balls melted in correct position to the processor. THEN you have to replace the chip on the board, and YET AGAIN bring it up to temp to melt the new solder to board (like reflow, this would just be a 'flow' I guess lol). Oh, yeah, you usually do this to the gpu AND cpu to cover your bases at minimum. You might do the ram too. Count on the reball kit running about $150 (includes stencils, balled solder, flux, and other tools). With this purchase, you could probably reball multiple processors (and that's about the only way to make this worth your while cost wise!). Video showing process is at http://www.youtube.com/watch?v=97nxZwHG5bA
guy does it from start to finish (chip only) in about 10 min, but an amateur will probably take much longer.