I hesitate to call it an hPDA, but I have a stack of index cards with NAs and such written on it. I have a Moleskine Pocket Notebook that I use for a journal, so the pages don't have much to do with GTD. I have contemplated getting another one to use in my GTD setup, but funds are tight right now. In the back pocket, I keep my index cards. The face of the pocket (what would be the inside back cover) has around 20 Post It flags and the gussets have a paper clip on each folded edge. The opposite face has several Post Its, usually around three to five regular sized ones and several smaller ones. I use this area for capturing. I use blank note cards for beaming. I've been meaning to post some pics to Flickr about this. I'll post back if I actually get around to it.

This isn't personal experience, but I know a lot of people use their Moleskine well in their hPDA setup. Correct me if I'm wrong (and believe me they will!), but it seems the pages are useful for getting ideas out on paper. Maybe it's brainstorming a domain name or hammering out criteria for a project. That sort of thing. Check out the Moleskine Hacks on 43F. Lots of good stuff there. Emory has a good Flickr set on his hPDA setup that uses a Moleskine Memo Pocket.