Recently replaced 4200 with 7200.
As my post title states, I recently replaced a 4200 RPM drive (40 GB Toshiba) with a 7200 RPM drive (60 GB Hitachi.) This is on a Windows laptop, and in a Norton 'system benchmark' it scored about 10% faster. (That's 'full computer' speed. All other specs were the same, so the drive alone giving a 10% boost is pretty good. By comparison, replacing the 1.4GHz Celeron-M with a 2.26GHz Pentium-M gave it a full 50% boost.)
But, for 'everyday' tasks, it FEELS way faster. It boots faster, programs load faster, it's just overall more responsive.
This was going from 4200 to 7200, though. I've never had a 5400 RPM notebook drive, so I can't reasonably compare. I'd say you'll see SOME improvement, probably noticable, but not nearly as drastic as I did moving from 4200. Also, if you're getting the low-end model with 512MB of RAM, increasing that to 1 GB may give you an equal improvement in speed.
I ordered a 1.8GHz MBP with 1GB RAM and the 7200 RPM hard drive.