"dave"... yes, you can use a PDA and a Tablet PC to draw diagrams so I think you are in the right thread. One of the reasons the Tablet came about was to capitalize more on how most people "think with paper and pen". I have used and owned Palms, PocketPCs, and a Tablet. Both have either built in or 3rd party programs that allow you to sketch. PDAs for drawing have average performance in my opinion. One of the limitations is that the screen is a bit too small to be an effective drawing surface.
While a tablet PC is not a drawing tablet per se, a tablet PC does have full sketching ability. The "Journal" function allows you to sketch much as you would on a piece of paper, and 3rd party programs, such Corel Grafigo are quite advanced and allow for sophisticated drawing, painting, and illustration. The previous response by netyan suggested Wacom tablets; in fact, many of the tablet PCs use Wacom technology.
However, if you want to sync drawing type information into your PB, you're going to have to look at 3rd party syncing software and see what can be transferred over.
For example, when I use a Tablet and if I make a freehand diagram but want somebody else to look at it who doesn't have a tablet, I can have that person load Windows Journal Viewer, which is a free viewing program not unlike Adobe Reader. The person can see the document but cannot change it. However, I don't think there is such a viewer for Mac, so you might have to run it in VirtualPC or save the file in the Tablet to something cross platform (PDF?).
As for a PDA, I'm not so sure. If the drawing is in a Notes file on the PDA (PocketPC), and if Entourage can handle Notes via 3rd party software (e.g., MissingSync), then the drawing should show up on your PB. I don't think you'd be able to edit it however - it would be strictly view only. I'm less sure about this as I'm in a similar situation and am contemplating moving everything to the Mac.
Good luck.