Paul Thurrott's a big expert on all things Microsoft. He has a great site with Supersite and knows a lot of details on the happenings at Redmond. This past weekend, he was taking on What The Tech and revealed what he does know on the next Xbox.
For starters, he says it will just be called Xbox and that it will retail for around $500 with a $300 option for some kind of a subscription mode. It will have Blu-ray and he has seen documents on a requirement for Internet connectivity, but don't know specifically what that means.
Another thing mentioned was a $99 Xbox 360 codenamed Stingray will also be a new SKU for Microsoft. If so, I'd pick one up just for it to be a Windows Media Extender.
Again, we shouldn't all be freaking out yet on what's not been announced or shown. Paul says that there will be a reveal on May 21, originally scheduled for April 24th. So, we got a month and change before we should finally get some concrete details on the next Xbox.