Well I am assuming you mean the base 2011 iMac vs the top end 2010. The top end for 2010 was the 2.93 i7 870. The base for 2011 is the Sandybridge i5 2500S at 2.7Ghz.
The top end iMac i7 2.93 from 2010 is the second fastest 27 iMac right now. Faster than the SandyBridge i5 3.1 2400. And yes it is faster than the base iMac. Much Faster.
For those that are saying Sandy Bridge, yes against the i5 2.8 i760. The i5 2.7 2500S is faster. Against the i7 860 2.8 the top end iMac from 2009 which was the top end BTO is faster than the current base 27 iMac. Heck, it's even as fast as or faster than the current top end retail 27 Sandybridge i5 3.1.
Hyperthreading is your friend.

After owning the i7 I would not own a Mac without one. Can you say future proof? A 2009 iMac 27 i7 is probably still the third fastest 27 iMac available.
2011 Top BTO:
http://browse.geekbench.ca/geekbench2/view/462699
2011 Top:
http://browse.geekbench.ca/geekbench2/view/460762
2011 Base:
http://browse.geekbench.ca/geekbench2/view/462599
2010 Top BTO:
http://browse.geekbench.ca/geekbench2/view/458086
2009 Top BTO:
http://browse.geekbench.ca/geekbench2/view/460086
You would be crazy not to get the 2010 top end model the i 870 2.93 for $1500.00. That is the best deal of and iMac right now.
I bought a refurb 2.93 27 with SSD. When the 2011's came out. Bought a few 2011 iMacs a 27 base with the 2500S with SSD and a 21.5 with a Sandybridge 2.8 i7 and SSD.
My current 'home' iMac the 2.93 i7 27 with SSD is about as fast as the 21.5 inch 2.8 with SSD I had before. It is faster than my 27 2011 i5 or 'base' 27 with SSD.