Right now I'm still planning an integration strategy for MSTest. It's a bit of a monster compared with the other frameworks, so I'd rather be certain of what I'm getting myself into :)
My best guess right now would be within the next few weeks. I'll try to announce it in quite a big way, so if you follow me on twitter or check the blog regularly then you'll definitely know when it's in there (or getting close!).
I do definitely have long term intentions to introduce pluggable test frameworks. Maintaining every testing framework under the sun on my own is definitely not how I'd prefer to spend my time ;) Of course, the priority at the moment is making sure that the system is stable, enjoyable, and accessible by everyone.