Well, shit.

X-Clamp Fix - This is the only legitimate fix I have found thus far. It is as permanent as it gets. Do not use it however if your Xbox 360 is still under warranty. Send the console back to Microsoft and let them honor the warranty. However, if the seal is broken and/or it isn't under warranty this fix is your best bet. This is the fix in a nutshell; you open up the console, remove the x-clamps, clean up the old thermal paste, put new thermal paste on, replace the x-clamps with screws and washers, reassemble the unit, overheat, let it cool down and it works on approximately 90% of the units. Recommendation: Use this fix.

Article Source: Xbox 360 RROD - Which Xbox 360 RROD Fix is the Best?