The Unoffical Apple Weblog had the scoop this morning, stating several sources claim Apple will supposedly show the new operating system to the public for the first time at the coming Worldwide Developers' Conference, including a developers' seeding. It won't contain any major new features, but instead it will focus on speed and stability, while also dropping support for anything but 64bit Intel machines. That's going to be rough if you bought a PowerMac G5 Quad.
Lending some credibility to the rumours, Ars' sources confirmed TUAW's findings.
A possibility is that Snow Leopard will be Cocoa-only, but this is apparently not yet set in stone, and what exactly "Cocoa-only" means is up for debate. Fact is that Carbon is not available in 64bit flavour.
Despite Ars' confirmation, this is still a rumour.