my understanding is that the Core 2 Duo was surprisingly good. Many experts were suggesting there would not be much of a jump, but it turned out it was quite noticable, at least in benchmark tests -- some tests 15% faster.
Having said that, many tests were 8%, some exactly the same. And in real world tests (not benchmark tests), there is little-to-no noticeable difference.
here are some graphs that show the difference in speed between MacBook Pros with core2duo and santa rosa core2duo. Yes, the newer chips are faster, but we're talking seconds here, and I think RAM is the biggest issue with Photoshop and similar applications, not processor.
Mac Buyer's Guide recommends buying as the MacBooks were updated fairly recently.
I guess the bottom line is if you're not in a rush, and you want to feel secure about your purchase, then wait, but if you do need it, then buy. Even if they update it tomrorow, I honestly don't thinkt here will be a noticable difference.
The main issue is Leopard. If you buy now, you will have to buy Leopard and/or iLife separately (assuming you want to upgrade), but if you wait until October, they will come free, saving you ~$200 or so.