I use a metal business card holder with an attached pen.
It holds a 2" x 3" mini-notebook, and comes with a pen which is attached and is also 3" long. Inside, I replace the mini-notebook (it also comes with a mini address book that fits inside) with post-it notes that come in the 2" x 3" size (can buy them at any Staples or Office Depot).
This metal card holder is very small and thin, the external dimensions with pen attached: 3.4" (L) x 2.4" (W) x 0.2" (D) and costs $9.50 (I'm not affiliated with the company selling this, in any way).
When out and about, when needed, I take quick notes on a post it note, and when I come back, I throw the post it's with notes into the inbox.
That takes care of the note-taking.
I also carry a 4GB iPod Nano 2nd generation, for my lists when I'm out and about. Before I leave, I load up my Nano with an .rtf file or files which I export from the iGTD application. Then, when I'm out, I can quickly scan the right list (@ appropriate context) of Next Actions. Additionally, you can buy a microphone plug in, and record voice notes if you have more extensive things to capture. The nano is very small and thin.
And that takes care of the lists.
This way I'm covered both ways when away from home. I was hoping for a way to combine both lists and note-taking in one device, but I never found the right tool. I looked into the smalles PDA I could find (the Palm Z22) but all electronic devices suffer from one big problem: input. It just is not convenient to take quick notes on them. So, for note-taking, I found best pen and paper, and then I looked for the smallest most convenient tools (I hate big things in my pockets). Conversely, the problem with carrying around Next Action Lists on paper is that it's hard to generate them from the computer - you either have to write them by hand or print them - both very awkward. It's much faster to load up a file to my iPod Nano.