I don't know about "some people", but my 6800 has been unlocked and used daily for over 4 months with no problems developing. Any video card can fail at any time, and I haven't seen evidence that unlocking pipes with a softmod is necessarily going to make that more likely. My earlier comment about checking oc on a 6800 after the softmod was intended to test whether the unlocked pipes are defective, regardless of whether artifacts are visible in every application. Most people who report that RT will not overclock after the softmod are having the problem because the unlocked pipes are defective - maybe not grossly, but defective just the same - and the sensitive artifact tester in the oc section fails every time and, therefore, will not let you apply an overclock. I have unlocked 4 6800's and only 2 were free of defects and could be overclocked normally, just like a card which hasn't been modded. One of the 2 w/defects actually only showed artifacts in one game of 3D03 and none of my other apps, but still failed the overclocker's artifact test.
All that said, based on owning and using both cards I still think that the X800pro is a better choice at a similar price (except for the D3 fanatic) and mine, even when operated at 12 pipelines, still performs better and smoother than my 16/6 6800 in my games (FC, HL2, NFSU2,FS, CSS). The 6800 is still a great card and, if unlocked, is really high in performance per dollar. It's only beaten here because you apparently have access to a really good deal on the X800pro.
Whichever card you choose, I'm sure you will enjoy it.