I just bought an MBP 13" last month to take advantage of a deferred payments option.
Sandy Bridge was the "tick" of Intel's "tick-tock" upgrade strategy and "ticks" are where the major jumps take place, with the "tocks" being improvements to the processes via a die-shrink. So Ivy Bridge will likely not bring any massive improvements to what we have now with Sandy Bridge with the exception of Intel's tri-gate transistor technology.
If I had to guess, I'd say USB 3 would probably be the "major" update for the 2012 MBP 13 upgrade, as that support will be in the Intel Panther Point chipsets by then.
I do not believe we will see significant improvements until Haswell arrives on the MacBook Pro in 2013, so I bought now.