I second the SanDisk Cruzer mini. It cost $45 with rebate and has been great. I also have a Micro Advantage 128MB flash drive, but it draws too much power to work on the Pro keyboard USB port and has to be plugged directly into the Mac's USB port. On the contrary the Sandisk Cruzer works perfectly everywhere I've tried it--OS 9, OS X, and Windows 98.
Chris