I use a Treo 650 + paper system. I haven't worked it all into the GTD framework, but it serves me well. After starting GTD, I tried using paper for all my to do items instead of putting them in the treo. At first I was using a legal pad. I found that I was more likely to write something down if it was on paper vs. on the smartphone (still had a Kyo 7135 when I started the system). From there, I started using the pocketmod instead of the legal pad, so that I'd always have it in my pocket to write on. The paper gets shredded pretty quickly, though, so I got a Levenger international pocket briefcase for my birthday and bought some blank 3x5 card and ones with the vertical (portrait-oriented) stripes at Staples and have been working with that ever since. Having the paper for random thoughts and to-do's works better for me because 1) I am more likely to write them down and 2) they don't have to fit into the data format for a Palm (such as having a date).

I find that this also makes my calendar more effective (I use Datebk5) because I don't have a swarm of to-do's crowding it out on the daily view. I use the Treo for contacts, definite time/date items that I want to get the reminder alarm for, and keep notes that need to be persistent in the Memos (like information for appealing a parking ticket or a rebate form). I also use Datashield to keep track of passwords, logins, frequent flyer numbers, and credit card information (so that I have the 800 numbers handy), and Rick Whitt's Directory Assistant to save money on calling 411 (can't recommend this one highly enough).

Since most of the to-do's I write down are time-flexible, using paper for them works out better for me, and I have the tactile satisfaction of the pocket briefcase with paper when I am writing out lists or ideas. I do still use Diddlebug on the Treo for some quick notes, like if I need to write down a phone number or address to get transferred later (that way I don't have to worry about punching it into the software wrong and it's still on the same device that it ultimately needs to end up on).

I have always read that you should stick to one system (PDA, day-timer, Outlook, whatever) or it won't work, but to me the most important thing is to find the most "friction-less" way of organizing your info. If doing some things on paper feels more natural, then the system will work better if you go with the flow even though you've broken up your system into digital/analog, IMHO.