I have a Palm Tungsten 2E that I got as a hand-me down and it isn't TOO old, really. It has its functions and it is a handy thing to take notes on in the store or at work and I use it for my car MP3 player. The problem, at least with the model that I have anyway, is that the stylus calibration is completely borked. No matter how many times I calibrate the touch screen on it, it's highly inaccurate. I did have a drawing program on there called Moe Paint (didn't have an english speaking developer and they suddenly stopped working on it at some point before I had even learned of its existence) that seemed like it would be something fun to pixel on, but with the terrible touch screen problems it was just annoying. Not to mention that at some point the program stopped working completely.
So you might find something that might be decent for portable pixeling. But I doubt it will ever be comparable to getting an old used tablet online in terms of the value of use you will get out of it for pixeling. Because there are so many new tablets out there like the awesome Intuos 4 (I finally sprung for a small version of it for my birthday last year and I looove it), you can find old Intuos 2 models or other similar models for pretty cheap on eBay sometimes. I sold my 6x8 intuos 2 for $40 to a friend when I got my new one. It was plenty perfect for pixel art needs, though.