David,
Definitely pull the trigger now, while you still have your finger on it.
If you have a good deal now, and you feel that the current Intel iMacs will fulfill your needs, then I would suggest you go ahead and get it now. With the exception of some professional apps that are not yet in universal format, these new iMacs are very fast, very efficient and very stable.
With regards to a rev b coming up, well here is what I have to say: Software changes very quickly, but I noticed that people's needs don't do so as fast. With the exception of work stuff (come to think of it, even with that), my personal needs have not really changed in the last 4 years---some photo editing, music, surfing, word processing. In that amount of time imacs have moved from the "lampshade" flat panel models to the current iMac Intels, and to tell you frankly, the iMac flat panel of four years ago will still fulfill my needs now (even on a professional level, actually).
What I'm saying is that another "newer" model may come up eventually (That's inevitable), but the Intel iMacs will not be outmoded with regards to your "needs" any time soon.
In fact, with Intel being a new platform, you can only expect forward movement in terms of new products, software and services in this area.
As to when the rev B will be coming... well, who knows. So if you've got a good deal now, go for the Intel iMac---it's the newest one out there so you can't go wrong.
Hope this helps.