I'm typing this on a new MacBook Pro 15" 2.4GHz. And I have to say it's fantastic! A wonderful upgrade from my previous 12" G4 1GHz PowerBook! That said, there may be an announcement in June but I wouldn't hold my breath. And if it is a new design please remember that it will be a rev A, and therefore you will effectively be a beta tester.
I can't see that any new design would be radically different, considering the MacBook Air is not so different to the MacBook Pro in terms of design. It may get thinner, but then part of that is hardware constraints - look at all the compromises that had to go into the MB Air! - and it's not as if the current design is a brick either. I love mine.
Still, you might as well wait a month or two if you can afford the luxury. I can't see that they will be considerably faster until 2009 though.