Appleinsider has heard that Apple will refresh its iMac line in the next few weeks. They cite unamed sources who say unamed upgrades will be coming. They do think that the iMacs will become thinner (why?) and "classier" (what?). They speculate that there will be Blu-Ray and some audio improvements (Dobly surround?) as well.
All iMacs are currently selling with an under 24-hour lead time at the Apple Store and only command a $30 or so discount at Amazon.