The turbo boost speed (speed achievable when some cores are idle) is also a bit higher. (maximum Turbo clock speed for the 2.2GHz is 3.2GHz, for 2.3GHz is 3.4GHz.) It also has a 8MB L3 Cache vs a 6MB L3.
So, I paid $230 (with corporate discount) for 2 more meg of Level 3 cache and 200 MHz higher peak speed. Not sure if it's worth it, but sounded good!