Well the review of the GTX 275 vs HD 4890 links here....

Mystery solved?
Alright, now that I've read the review, it seems like they trade blows all around, doesn't seem to be a clear cut winner here. The 4890 does have the ace that its hard launch, but it seems to be a high power consumer while idle, sure its load consumption is GREAT, but most people spend their time idling their video cards. Long story short? Its a mixed bag on both sides.
My opinion? It comes down to what type of games you play, and your budget. It really seems to be split down the middle, with ATi better at some games, while the nVidia at others. In the end though the 4890 looks like it edges out slightly, even if only due to its overclocking potential(

), and the fact that
you can buy it right now(not a paper launch). Its an interesting refresh, on both sides, none the less.