There are quite a few reports indicating a number of defected iMacs around the web. This pursued me to shed some facts among puzzled Mac users.
Power Consumption In my opintion it should be obvious that the compact housing of an iMac cannot handle the raw power of the 8 threaded cores of the i7 in a 100% silent manner. The i7 sports 20 - 70 Watts more power consumption over the i5. The i5 which is not only lacking HT but it is also clocked lower about 4x130 MHz. When unthreaded the iMac Core i7 860 + LGA1156 should draw about the same power as the Core i5 750 + LGA1156. My research indicate that in Idle mode the i5/i7 is much more power efficient than the Core2Duo CPU. On full load the i5/i7 will need about 50 Watt more compared to the Core2Duo.
Fan Noise In my opintion 20-70 Watts more power consumption compared to the C2Duo iMacs leads to more heat. This would lead probably to more noise of the fans, if your heat managment concept isn't flawless. In my experience I consider that only with a DIY System you are able to apply a flawless temperature management. Because usually with a DIY system there are no budget restrictions.
Harddrive Noise There have been reports about a grumbling sound on the iMac. A spinning HDD will almost always vibrate and the computer housing will resonate that vibration. You can however lower or even kill that vibration with rubber or other dampening. I am almost sure Apple did that. But I am not so sure if Apple's solution kills noise with the most powerful HDD of the Caviar series -the WD Caviar Black- which has been built into iMacs (besides other models). I got the Caviar Blue in my DIY PC and dampened it. I found that it still grumbles a bit on activity when it's absolutely silent in the room.
GPU The iMac i7 is equipped with an ATI 4850 graphics card. The 4850 memory and GPU runs 100-250 Mhz clocks higher in idle and load compared to the 4670 of the iMac Core2Duo. It consumes about 30 Watts more power compared to the ATI 4670 (in idle). These findings however apply to the mobility GPU, which should consume less power. To clarify: There are 500 MHz Core and 850 MHz Memory clock on the mobility Radeon vs 625 MHz/990 Mhz on the desktop version.
The iMacs i7 860 (Lynnfield) sits on a 45nm die and is a very powerful CPU, in fact it is one of the most powerful CPU's you can get for a desktop computer today. One should anticipate some trade-off in terms of power consumption vs heat.
You might consider that the iMac i7 draws about 260 Watts at full load. The Mac Mini draws 18Watt -20 times less- but of course it doesn't come with the big 27inch display. However Intel's Lynnfield deliver most performance per Watt compared to any recent CPU lineup. Thus I shall let you decide weather the EPA ENERGY STAR qualification of the iMac is hence justified.
Update October 2010:
After using an iMac i5 for 6 months now I must say I am very impressed in a positive way. The display is absolutely brilliant, its black level is in my opinion only beaten by a Sony Bravia TV of latest generation.
The harddisk grumples silently and seems to be the loudest source of noise of the iMac. The ventilation is absolutely no concern. I can barely make it out in a silent room. When gaming in Windows7 the vents fire up but produce a gentle humming without distracting from gaming.
In my opinion these next-gen Mac's rock!