I wouldn't doubt a closed beta, like the one I am participating in for Halo First Offensive, some select professional games that don't neccisarly want to test the game, but can test the game and find bugs. Of course that is our job, and we are always bug hunting, for the most part. Never said we can't have any fun. =p
The beta, if there is one, will probably only be open for a select few on these forums, like the Halo 3 one, but without the rest being able to by buying crackdown. Sorry noobs, but just saying I want to participate in the Halo Wars beta isn't going to get you in, so yes there will probably be one, but no, not on your terms or image of a beta.