I've been thinking the same thing for the past month.
The iMacs where re-designed October 20th last year, so its been a little over 5 months since the have been out. With all the screen issues for both the 21 and the 27" models, i would expect a new update after a Macbook Pro and/or Mac Pro update, which would probably be around May/June (seeing as though the iPad will take up April) But who knows when they'll be updated.
All i really want apple to do is make a 21.5" i5.
I would snap that up as soon as it would be available 🙂