I just read that Waterloo has teamed up with Casio to bring a version of Maple 5 to Win CE. It has only been compiled for the one product, which has somewhat limited hardware: only 4 color low-res VGA. But it's a full CAS on a PDA!
Recent advances in handheld PC's are a great improvement over the first models in terms of features and speed. Hitachi has a tablet unit coming out, the HPW600ET, that supports 32MB RAM, an SH-4 128MHz CPU, PC Card II, USB, 64k colors w/ VGA Res (back-lit touchscreen), et al. It has no tiny keyboard that makes for awkward input. Rather, it uses handwriting recognition. It weighs about 822 grams (29 oz) and is about half the size of a sheet of 20 x 29 cm (8.5x11 in) paper. And with recent advances made by IBM in the field of mini hard drives and Iomega's Click drive, I see the forces brewing for yet another revolution in unbiquitous computing.
Does Wolfram Research have any plans to bring Mathematica to the Win CE handheld platform? If it doesn't, then it probably should start planning.