Well, just to make sure that it doesn't come to a surprise (either nice or shouldn't) if he needs one or not. But if he does, ask him. Yes, he should still be pleasant that you would get one for him. But if you prefer not to ask him...

If he is on the go, the must haves are: WiFi and Bluetooth (to connect to the computer or a cellular device). Also a nice memory chip, somewhere around 512MB or a 1GB. Memories on a pda itself aren't much at all.

If he likes to schedule (maybe not?), view multimedia, listen to music, etc., be sure to think about that when you decide your PDA. A great idea if you want your hubby to relax on his lunch duration. Unless he's an audiophile, I don't think an iPod needs an introduction.

If you are looking for a Pocket PC (operated by Microsoft), the best companies to look into are Dell and HP. Maybe I'm bias, but I used to have a Palm OS device, I didn't really like it at all, didn't have a fresh look on the interface.

The only other thing I can think of is a foldable keyboard if you see him type alot (but you may be in favor of not going this way, save his eyes from straining while trying to type onto a 4" screen) or a GPS Card (Make sure its compatible with the PDA you purchase or get a PDA with one built in) in case if he has to travel to some country where he gets lost. But for the most part we go in groups so no need for these extra accessories.