I'd speculate that Apple need to do another update in the next 2-3 months in order to put enough space between the new (sept) range and thier Spring Arrandale release, without allowing the current model iMac to go stale.
also the bombardment of new features seems unlike apple, a iMac from this year moving into next year and gaining Arrandale, Bluray and DDR3 seems a bit of a leap and a giant finger at last years model...
I'd have to agree, from what I have seen from Apple. The way they upgrade/update products is gradually over time. Look at the Macbook range.
The way the iphone has been updated yearly but gradually. First 2g then 3g and then a faster 3g. The imac will probably be the same, probably a bit quicker with one or 2 other small enhancements and a 7-10% price drop. before a complete redesign sometime next year.