Well, how latest?
I mean, WPMU 2.7.1 came out a couple hours ago. https://mu.wordpress.org/download/.
BP rc2 is likely to come out in the next day or two. BP rc2 and the current trunk and the v1 release will need WPMU 2.7.1. I suggest you wait for BP rc2, then upgrade to WPMU 2.7.1 and BP rc2 and see if the problem remains.
There’s been so much stuff fixed since rc1..