From what I know about software development beta releases are feature complete. (And all I know about are browsers; Mozilla and Opera so far.) This is not always true as Mozilla for example is currently adding support for a couple of CSS 3 user interface pseudo-classes while they are in late beta (beta 3), but mostly beta means fixing small bugs and crash bugs. So it is nice and all from the WaSP to defend Microsoft for a bit, but it does not make any sense to me.
I hope I stand corrected at the official release or beta 2 if it comes out, but they are not fixing a lot of CSS 2.1 before that I think. (Let alone other standards like — but not limited to — the DOM and HTML.)