Over X Input 1.5, X Input 2 brings support for multiple input master devices and the master/slave device hierarchy being visible to the client. All events in X Input 2 will also be backed by GenericEvents. For those interested in reading the specification, it can be read in this email.
Peter also mentioned that X Input 2 should be ready for X Server 1.7, which will possibly be out later this year and coincide with X.Org 7.5. Originally X Input 2 was set to be included with X Server 1.6, but that didn't happen and before that it was even on the table to be included with X Server 1.5. Now though it looks like X Server 1.7 will be the magic release and will also allow Multi-Pointer X to be enabled by default.