The iMac always has a very long product cycle. The first was in August, then in April this year, then my guess is the next will be in January or February.
What Apple did was use some Montevina like technology in the iMac, therefore if they did update them to Montevina there would be no change at all in CPU clock speed, RAM, etc. Or they might just leave the current technology in there until January. No point in updating in September only to change again in January (although they have been known to turn over products in 3 months with the Macbook, although the Macbook is there best selling product by far).
Basically what I'm saying is, buy it now, stop worrying about future releases.