> So has anyone got anywhere/had issues with FC5/gcc 4.1/x86_64/RT?
nope, but ive never gotten the RT series to work without either freezing/instant-rebooting right after grub passes off execution to the kernel, or a kernel panic on some ACPI or similar issue during bootup, or a random unprovoked hard freeze within about 5 to 15 minutse after booting up. this is on several different machines, a Sempron64, a Turion64, and an Athlon64, on VIA, MSI, and Shuttle platforms. somehow Mark Knecht i think has claimed stable operation on this architecture, so maybe you want to ask him his magical combination of hardware / compiler / kernel / patch.
> I'm sore tempted to go back to 2.6.14-rtXX, but don't know what that will
probably nothing. if youve found a version that works, by all means, use it!
> with the update to the compiler, the OS, etc.
shouldnt be a problem. youre much more likely to run into issues when removing a version of GCC or Glibc and not properly updating the .la files and the like..