I don't know. I've heard it won't work without substantial modding. Vague references to having to bend the heatpipe, and possibly remove one of the keyboard support posts.
I'm not convinced yet. I'm still waiting to correspond with someone that has tried it.
I called Dell, explained that I have an XPS Gen2 and was wondering if the newer 7900GTX was going to be made available as an upgrade like the 7800GTX is now... we clearly don't speak the same language. First guy tried to sell me a desktop card, second guy didn't know, and the third guy said that the Gen 2 could only support 256mb of memory and wouldn't work right with the other. It'd fit, but the computer would only see 256mb of memory, LOL.
I'm still looking for someone with real experience... it'll only happen once the parts are more freely available. Right now they are being very (VERY!) tightly controlled.