A Pentium M is blazing fast, but suffers from one drawback inherent in S478 systems--the lack of 64bit support. I'm expecting 64 bit applications to be readily available in a year's time, so I'm hesitant to invest much in platforms that can't support it.
That being said, a P-M is probably the best cpu upgrade for your current rig, if that's the only thing you are considering.
I like the LGA stuff a lot, you could buy something like an P5P800 or AS8 Abit and run a P4-506 at 4GHz and have 64 bit support in addition to good core performance for ~$200. The lack of HT is the most serious drawback, but is also inherent in Pentium-Ms. You could also run a P4-630 to good effect, and they aren't that expensive these days. It gives a 2MB cache, HT, and 64 bit support, and would only be a ~$300 upgrade.
Considering how cheap the LGA alternitives are, you really have to think about how much you want to invest in something specific to S478. You will want to get off of AGP sooner or later, and when that happens, the party's over with S478. Between that and the lack of 64 bit support I'm hesitant to spend $200-250 on a processor upgrade when a LGA MB and CPU isn't hardly any more.