I'm sorry, but I wouldn't pay for being a beta user. Being a hacker myself, I commit a lot of time when I sign up for a beta. I report bugs, and give my opinion on what I like and what I don't like. In my opinion, a beta should never be the paid version. In this case, I'd say 'limited beta' is just marketing speak for an early bird deal. Which is OK, as long as you call it that.
Founder Gina Bianchini (ex-Ning) explained on launch.is that it is about setting up correct expectations: it'll have a strong "premium" component to it after launch, so apparently they don't want anyone to expect different.
I find it an interesting move given the large number of beta signups pages these days, but wouldn't sign up for MB, either.
I agree with dirkdeman: make it free for beta users we provide feedback etc or change it to an early signup offer...the idea seems to very good indeed. It reminds me of the Game startup by Andrew Hsu:
http://www.andrewhsu.com/projects/gamestudio.htm.