It is Clarksfield, not Clarkdale, that is a possible choice for the iMac. It is a mobile class processor package.
The non top end Clarksfield are 35-45W TDP which conceptually would work (some power radiation is being moved around from chipset to processor). Would be somewhat of a stretch though running 4 cores (and some half dozen, or more, of GPU cores ) full blast inside of an iMac case. However, not sure if you could do a whole line-up of iMacs with largely the same motherboard if tried that though. Wouldn't make sense to have iMacs with vastly different boards. For example bottom two on Core Duo and upper two on i7/i3 designs. Not going to get maximum scale on parts.
Clarksfield was to be priced a bit high for iMacs though.
http://www.geek.com/articles/chips/...le-nehalem-clarksfield-cpus-in-2009-20090421/
Longer term it is Arrandale ( 32nm) is Apple wants to keep the iMac pegged at 2 cores max. Staying at 2 cores would also help them move the prices down. Those arrive has been pulled back from Q1 2010 to Q4 2009 ... but can the iMac wait that long for a refresh?