There appears to be a problem with the current generation 27” iMacs. Apple keeps issuing new firmware updates and, by some reports, has halted manufacturing. I’ve heard from several readers with complaints (and returns) of their 27” iMacs. While Apple hasn’t come out and explained exactly what is going on, tonight they released their second firmware update and clearly something is amiss. I think this calls both the 21” and 27” models into question.
I still believe in the iMac form factor and I’m sure Apple will sort it out soon enough. Most likely, my next iMac will be one of those 27” monsters. For the time being, however, I recommend waiting until the dust settles before buying a new iMac.