According to a source, there are currently up to six (6) enclosures undergoing internal testing. Features vary and it's unknown which features may make it to the final hardware... but some features include: white speaker, front mounted headphone jack, 4x512MB RAM, new motherboard, bays for two optical drives.
Certainly nothing earth-shattering, but of interest for those following closely... :)
MacWorld Expo NY is only 10 days away, and expectations are upgrades to the current PowerMac line.