ATI's HD4890 has been in the works for quite some time, and nothing is final about it. There's been rumours flying around left and right. Maybe Nvidia just wants some attention as well. The GTX275 most definately is a reaction to the HD4890. But I don't think the GTX275 makes much sense. I mean, we all know how close a GTX260 black edition can get to a GTX280, and even perform on par. A GTX275 will tread really close to a GTX280/GTX285. Also, there will be no room for a GTX260, since if only the clockspeeds of the GTX275 get jacked up compared to the GTX260, factory oc-ed gtx260 = equal to GTX275. It's very hard to imagine a GTX275 gets 240 sp's, so that leaves either a 512-bit memorybus, or maybe, but less likely, 240sp's with a 448-bit memorybus.
All in all, I don't see this happening. There just isn't much room for improving a GTX260, with the GTX285 allready out there. If it does happen though, and the GTX260 really lowers in price, and it beats the HD4890, AMD will most likely react again with another price cut. It's win win for the consumer, no matter what happens. CJ actually thinks the GTX260 will get phased out, so Nvidia has a 275/285/295 line-up. Oh, and last but not least, if Nvidia was to release this card within a month *6th of april* it must have the gpu's allready. Perhaps that makes a 448-bit 240sp-gpu more likely?