I am not a palm person but I do know of this piece of software called netchaser, it is $10 and only for the tungsten c (I think) but it is worth a shot. As far as ministumbler for palm os, couldn't tell you.
NetChaser fills the bill for the Tungsten C in peforming many of the important functions of Netstumbler. And, it produces a compatible datafile. Well worth the ten bucks. I have used mine for about a year since its early beta days.