Frank Carver, over at RaspberryAlphaOmega, has done a comparison between the different revision numbers of the two Models of the Raspberry Pi. He lists the different revisions and explains what capabilities or tweaks can be inferred from the revision number.

Automatic Raspberry Pi board revision detection: model A, B1 and B2 | Raspberry Alpha Omega.

This is an excellent companion piece to Daniel Bull’s recent MK Raspberry Jam thread.