I've not seen any performance issues so far. Addmittedly I've not installed any games yet that really stretch my comp. But I have been playing at 1680x1050 on full settings and getting great performance in games like CSS (100+ fps).
Don't play WoW so can't comment on that.
I'm going to be installing Doom3, so I'll see how that runs. I've also got FEAR, which I could benchmark.
Unfortunately I don't really have anything newer, as those games used to really push my old P4 system. I've only just upgraded and not been out to buy anything new yet.
Will happily run a few benchmarks if someone's concerned about performance - but so far I've not noticed anything obvious.
I would imagine that any performance issues that do exists would just be driver issues, that will be fixed over time. On a decent system there's no reason why Vista should be inherantly slower than XP.