Yesterday, Donncha released WordPress MU version 2.9.2. 2.9.2 contains fixes for the major outstanding bugs in MU. Unless a security issue surfaces in either WordPress or WordPress MU, this is expected to be the last version of MU. Thanks Donncha  

It’s officially out: WPMU 2.9.1. As Donncha says, this will likely be the last release before 3.0 and the merge. PLEASE see the info about adding the new commentsmeta table. (Wow, what a crazy ride. )  

Unlike the upgrade between regular WordPress from 2.6 to 2.6.1 being non-urgent, this upgrade for WPMU *is* required if you’re on 2.6. One reason is it fixes a really nasty bug with deleting blogs. You can check the trac timeline to see exactly what was fixed & changed.