The 2011 iMac will theoretically lose support one year earlier than the 2012 iMac. Thus, if you wait ~ a month, you will gain a year worth of support for no extra cost.
This is the best reason I have head thus far for waiting. But it only applies if you keep your machines for multiple years / past depreciation dates.
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If the iMac is going to have a silent refresh it's most likely going to be in the next week or two and just POP up on the store.
Respect your decision to wait (some have been waiting since October, ask them if it's been worth it). But one thing to note, Apple doesn't do quiet. Last year's 'quiet' MBP refresh told us this may be the last laptop we ever need!