Despite what others have posted, I totally understand what you're saying. The reason the most recent update was over a year after the previous revision is because the design is completely new. That's historically consistent for Apple and their new designs. I do agree that you should buy whatever is available today if you need a computer to do work/school/etc., but if you have a computer that satisfies your needs although it needs a refresh, you shouldn't buy arbitrarily because a new version is available.
I am in the market for a new iMac, but my current G4 iMac does enough of what I want it to do to give me flexibility about when I buy. I completely agree that waiting for Leopard makes sense. Why should you pay over $100 more if you don't need it today? Whether you consider next month to be mid-product cycle for the current iMac is certainly debatable. I personally think the switch to Intel processors makes Apple's products more predictable. For example, it was only a matter of time before Apple included Santa Rosa in the iMac. In my opinion, the delay between Intel's Santa Rosa release and Apple's release of the iMac was a big indicator of the redesign.
Again, it's only my opinion, but I think the next iMac revision won't come until Mar-Apr of next year which will coincide with Intel's release of the Montevina chipset. Although Apple could drop the Penryn processor into the iMac in the first quarter of next year, I don't think that is likely because Apple typically goes at least six months between revisions (you couldn't recoup design costs with shorter release cycles unless you're design was a beige box). It's hard to say whether a new GPU will be available. Apple doesn't generally keep pace regarding graphics cards in its consumer/prosumer models. Additionally, the second revision of any product for any company tends to have fewer bugs/kinks.
So, back to your question, Phtosynthesis. If you need it today, buy it. If you don't and can't wait until Q2 08, buy it when Leopard is included. Otherwise, spend your money on something else and don't worry about it till next year.