Are the CPUs desktop in the new iMac?

Yes, they are desktop CPUs.

http://forums.macrumors.com/showthread.php?t=1474557

The GPUs are mobile, but very powerful nevertheless.

 

The lines are blurred between "desktop" and "mobile" Intel CPUs these days. There isn't any difference in performance if you take a "mobile" and "desktop" CPU with the same specs. It's just the desktop version will cost less and have a higher TDP rating.

It's not like GPUs where you sometimes have mobile and desktop versions of products which are actually different, but have similar and confusing names.

 

I would disagree. The lines are very clear on desktop and mobile CPUs. It's not just about costing less and a higher TDP rating. It's also about a higher base clock rate, a higher turbo boost, Streaming SIMD Extensions 4 support.

It's also bigger.

 

OK, but note that I said "with the same specs".

If you take, say, a "Standard Power (77W max TDP)" and "Low Power" (65W TDP) Ivy Bridge part at the same clock speed, the performance will be pretty much identical. It's just the Low Power parts cost a bit more.

A "mobile" Ivy Bridge part really just means it's Dual Core instead of Quad, which more-or-less halves the TDP. A desktop dual-core Ivy Bridge is the same, performance and TDP wise, as a Mobile part - just packaged differently.

And I'm pretty sure all Ivy Bridge parts support things like SSE4 - even the Ultra-low-power versions.

 

Nope, they only support AVX instruction set extensions. No SSE4. Check out ark.intel.com