I've flashed my PNY 6800GT -> PNY 6800Ultra. All it did was show up as an Ultra card in Windows (had to reinstall drivers) and run more hot (ultra seems to have higher voltages)
Before flashing, though, I edited the ultra bios to match GT clockspeeds, I didn't want to overclock yet and I wasn't sure if my card could even handle ultra clockspeeds.
To sum it up, I don't recommend it, unless you want to try to raise your vGPU and try to overclock more than is possible with stock voltages.