Have to call BS on this - the reason Apple isn't using Arrandale in the 13" MBP is the same reason they waited so long to introduce it in the 15/17", and the same reason they raised the price of the entry-level 15", and the same reason they made the default CPU on the 17" a 2.53 instead of a 2.66:
Chip supply.
Apple clearly came to the conclusion that they're not going to be able to source enough i3s/i5s from Intel right now to rely on them for their most popular system. So they use graphics and battery life as convenient excuses for not upgrading, then 6 months from now, when supplies are less constrained, they'll do the upgrade and tout the improved performance. ("our fastest MacBook ever")
Likewise, the 17" gets a $200 price cut and a midrange CPU because they'd rather ration their limited stock of i7s for the people who care enough about them to go to the trouble of special-ordering and paying extra; many graphics professionals will be happy with 2.53 GHz as long as they've got the new GPU and the big screen, and while under ordinary circumstances Apple would rather wring that extra $200 out of them, in this case they don't have the chips.
And the entry-level 15" gets a price increase because they don't think they'll be able to get enough i5s for the number of units they'd be able to sell at the old $1700 price point.
So it's all just a chip supply problem, plain and simple - they had to do some sort of update now, but they're not going to be able to get enough i5s/i7s to arrange the product line in the way they really want to until later in the year.