There were a lot of questions about why Apple didn’t put the i5 in the 13 inch machine. My theory was that they wanted to separate the high-end 13 inch and the low-end 15 inch. Prior to the new machines, the high-end 13 inch and low-end 15 inch has the exact same specs except for screen size, but cost $200 more. I suspect that the the more expensive machine was getting cannibalized. But apparently, my theory was wrong:

In the end, engineering constraints led Apple to stay with Core 2 Duo processors in the 13″ MacBook Pro for now. Cramming an i5 CPU into the chassis would have been possible, but it would have required sacrificing either graphics performance or taking a huge hit to battery life. Instead, graphics performance for the 13″ MacBook Pro has improved by as much as 1.8x the performance of the 2009 model, and the 10-hour battery life is on par with the iPad, which is pretty stellar performance for a traditional notebook computer.

This makes sense to me. Chris Rawson goes on to say that it’s very likely the 13 inch with get the i5 with the next update.