why wouldn't an x86 build be plausible?
Rhapsody had an Intel build up to DR2, and apparently after that. There's not much change in the APIs between Rhap DR2 and 10.0. 10.1 changed some things, but I'm sure that if they'd kept them synched up to that point, then it'd be easy to keep them source compatible.
But... an x86 version of OS X won't ever run Classic or Carbon apps. Carbon still makes PPC native calls, which is why it runs on OS 9, while Cocoa is abstracted through APIs enough to be portable.
To be honest, I'd rather see the classic OS go open source more than I'd like to see an open OS X.