Try not to think of it that way. Don't ask yourself, "am I going to have the most powerful iMac anyone in the world can possibly have for the longest period of time possible?"
Ask yourself instead:
1. How much will my life be improved by having this machine now vs. having it later on?
2. How much useful life will I get out of this machine if I purchase now?
In all honesty, if you're getting an i5/i7 iMac today, it's going to be very powerful for a good long time to come. A 2006 MacBook can run Final Cut Pro 7 respectably. Certainly, it won't run it as well as a 2010 MacBook Pro, but it's still a decent machine. I know this from experience.
If having it now will help your computing goals, get it now. It's not going to be less powerful just because a more powerful machine came out later. Not to mention, there are already faster machines on the market. Are you agonizing over the fact that you're looking at a 27" iMac rather than 12-core Mac Pro with a 27" LED Cinema Display? I doubt it. Not to mention, you won't waste months of your live agonizing over when you'll be able to get it. While others wait, you can spend this time enjoying your new machine. The only difference between buying at the end of a product cycle versus the beginning is the wait.
On the other hand, if having it now won't really help you, why buy at all?