according to several sites and rumors, apple is going to renew the look and spec of the old known iMac, this supposedly should happen in mid of october this year (15/10/2009), the last rumor is comming from AppleInsider:
http://www.appleinsider.com/articles/09/09/24/apple_ready_and_waiting_with_redesigned_imac_line.html
the rumor say that the current iMac models prices will be reduced by 100$ - 200$, regarding what is comming for the next model starts with blue ray drive, sd-card slot, multi-touch support, slimmer chassis, and what not, but the history shows that eventually the changes were much smaller like CPU upgrade or bigger storage capacity,
The last time Apple updated the iMac was in early March 2009, when the company doubled the amount of RAM, increased storage space and dropped in faster Intel Core 2 Duo processors while keeping prices stable for all but one model, the least-expensive 24-inch iMac. (InfoWorld)
keep checking...