I've designed several databases to run on both Windows Mobile/CE and the Palm OS. All of these seem to lack in some way (especially for development). Software companies like FileMaker have developed lightweight (watered down) versions of their product that will run on a pocket device. The downside is that you have to synchronize the data with the "mother ship" database and the process can be quite tempermental.
I'm coming around to the idea that a simplified XML schema for databases might be the way to go on portable devices. It would allow for data collection on any device, and be importable by any desktop database.
AA