I had previously stripped my Xbox, which is fun in itself, and borrowed a hot-air reflow blower from work. I removed both heatsinks and cleaned the old heat transfer compound off of the processors with cleaning solvent.
I had a large lump of aluminium round bar sitting on top of the GPU to hold it in place and put some downward pressure on it while I attempted to reflow it. After letting it cool, I refitted the heatsinks; I've only got some of the traditional white heat transfer paste, so I used it, instead of the silver original paste which is probably something similar to "Arctic Silver" or whatever it's called. I plugged in the power supply and AV cable, and powered-up... nothing; RRoD still smiling at me.
BTW the heatsink mod found on the net; didn't appeal to me because the alternative heatsink clamping method wouldn't put pressure in the centre, essentially allowing the PCB to flex and a gap to form between the top of the GPU and the H/S. The original x-spring clips have a centre "point" to prevent it. Also, the suggestion of running the GPU with no cooling or wrapping a towel around it or similar in order to heat the processor up enough to melt its own solder joints sounds like a sure-fire way to knacker it. If you don't kill it straight away, I'm sure you'd reduce its life.
I was deciding what to do with my dead Xbox, now that I couldn't save it. I was about to pack up the reflow gun, ready to take back to work, when I thought I might as well give it another go.
This time I left the H/S on and attempted to blow the hot air under the processor. I gave it a little bit of extra time, because of the H/S. I also thought I should do the CPU as well, it may not be (only) the GPU. Then I left it to cool.
Naked zombie Xbox on trial.
I dropped the drive back in, reconnected the leads and hit the power: the fans ramped to full power (the H/S were still very warm) and I was treated to the usual dance of green lights! As I type, it's been on for about 4 hours playing DVDs. The real test will be when I re-case it and hit it with a 9 hr gaming session!!!