Here's my fix which doesn't involve towels, heatguns or DIY reflow machines.
Had my first RROD (0020) yesterday, not bad considering the 360 was pre-owned, and i've had it for well over a year.
I bought a faulty 360 a while ago because I like trying out mods on broken shit, that way if I mess it up, nothing's lost. I decided to use the spare CPU heatsink from it on the GPU.
I gave the board a good clean. The RAM chips on the top had what looked like dried puddles of flux around them, so I used a spray solvent to tidy it up, and blasted some under the CPU & GPU for good measure.
Removed the old thermal paste, polished the CPU, GPU and heatsinks with a microfibre cloth before re-pasting and fitting them.
I let a small amount of flux run under the GPU, not sure whether that would help fix any cold solder joins.
After gently preheating the the board in the oven, I switched it on with a fan cooling only the CPU, leaving the GPU to cook.
I'm not sure what temperature would be dangerous for the GPU, I switched it off once the heatsink became too hot to touch for more than a second, which was around 15-20 minutes.
I left it to cool down then switched it back on and it worked. I've tried it this morning after leaving it off overnight and everything seems ok for now.
I have a cooling kit which I never use, so I pulled the fans from it, which fitted nicely on the back of the heatsinks, and are wired to run at full speed. They're held in place with sellotape, i'll need to find a better way of securing them.
I've also covered the top of the heatsinks, to make sure that air is being pulled through full width of the fins.
Here's the chemicals I used which cost £20:
Akasa TIM Clean thermal paste remover
Akasa AK-455 thermal paste
Servisol Aero Klene 50
The only problem with the GPU heatsink is, the DVD-ROM won't fit in the case. Gives me an idea about using an HD-DVD case to house it once I put it all back together again.
As long as it lasts until I get paid, i'm happy :-)