After two weeks, it is finally time for MAME 0.128u4. But be warned: this version contains some significant changes internally. The end goal is to make MAME more "object-oriented" which in turn will allow MAME to context switch between CPUs at a faster clip. Unfortunately, due to MAME's legacy architecture, this ends up being a somewhat painful change.
If you're a coder and inclined to make or suggest sweeping changes, this would be a bad time. However, if you'd like to help out in the changeover, you can help out by working on removing all references to the "active" CPU, as mentioned in the whatsnew.txt file. If you do opt to pitch in here, please coordinate with the rest of the developers by submitting your intentions ahead of time, to avoid duplication of effort.