At the end of last year, customers who bought 27" iMacs reported screen flickering. Apple acknowledged the problem and released a firmware update intended to fix it back in December. Unfortunately, the problem persists.
According to Ars Technica, Apple's internal support system is aware of the continuing problem and working on a fix. Customers who complain about the issue are to be told that their faulty display can be replaced "...in about three weeks." Note that the units currently for sale have a 3-week shipping time, so you might want to wait a month before making a purchase Ars suggests, and we agree.
The shipping delays began in December when Apple declined to comment on why, but discussion threads pointed to the display. The delay was pushed ahead even further in January, which is how we got to the present 3-week wait.
Here's hoping this gets fixed soon and all you owners can enjoy your 27" iMacs in all their non-flickering glory.