Looking at the difference between the core i5's in the 15 Inch, I am not seeing much difference:
Passmark CPU benchmarks
(higher is better)
Intel Core i5 M 520 @ 2.40GHz - 2492
Intel Core i5 M 540 @ 2.53GHz - 2566
Huge bump when it comes to the i7
Intel Core i7 920 @ 2.67GHz - 5589
Not so hot with the 13 incher, but not horrible.
Intel Core2 Duo P9600 @ 2.66GHz - 1976
(not sure which core 2 duo they are using, this is the fastest one out there)
I am guessing if they put a core i3 in there it would have been one of these two:
Intel Core i3 M 330 @ 2.13GHz - 2039
Intel Core i3 M 350 @ 2.27GHz - 2084
for the small performance boost over the core 2 duo that the i3 would have given, it doesnt seem worthwhile for the trade off in gpu...
Got the numbers from here:
http://www.cpubenchmark.net/high_end_cpus.html