THAT is why I went with the Handspring Visor. I got to save the $ of buying a USB adaptor, and I have the Springboard slot for an easy upgrade if my 2MB ever gets filled. Anyway, as for the m100, there is no "standard" way of upgrading the memory. However, like all hardware, it can be hacked.
http://www.go2mac.com/displaynews.cfm?newsid=7281 has a link to a page with the instructions. Note, however, that this requires opening the unit and messing around with soldering and such. Also, it voids your warranty. If you're not that adventurous.. well.. I guess you're out of luck.