There have been, so far, 2 firmware updates to fix the flicker issues. Delays on the production line to hopefully correct any hardware panel issues related to the yellowing of screens, but that hasn't been confirmed.
The latest reports say that the delay on the 27" has come down from 3 weeks to 2 weeks..and Apple's story is that the demand is too high for these.
There are lots of people who've bought the 27" iMac and have had no issues, while others who are on their 3rd iMac waiting for it to work. The latest change with the replacements however is that Apple has instructed their support team not to just keep replacing the iMac's in hopes that the new one will work..they're actually suggesting customer return the iMac for a refund..
In light of the ongoing problems, it makes sense to wait. I wouldn't want to spend $2000 on a machine that didn't meet my expectations plus have to deal with possibly returning it. It'll sort out in the near future. Apple has too much invested in not only the machine itself but also its own reputation.
I had the flicker problem on mine, which I purchased online the day after Christmas. I was about fed up with the flicker problem just when the update was released and the problem is now totally resolved. I couldn't be happier. Love my 27" i-7!