Well the "Launch" of the GTX 275 is in full swing as the curtain goes up on the NDA. These cards are not yet available in the channel but you can take a look at how the press samples perform to get a rough idea of what you will be getting with the new boards.
Most of the reviews we have read put the HD 4890 and GTX 275 in about the same performance class with the HD 4890 running slightly cheaper.
We are working on our own coverage of the GTX 275 and will be publishing it soon.
We will add more as they show up.