For mobility and utility, the "notebook" style computers are the best for doing both. The bummer is they are not as inexpensive. The Macbook Air is the strongly prefered solution at my workplace. Light, instant on, able to display and model on but very easy to tote around. A netbook comes nowhere close to the same utility, had one and it wasn't enough to actually do what I needed and was way too slow to really be useful. I ended up bringing my laptop instead.
Although tablets are not there yet from a brute number crunching standpoint, they will be soon. The iPad is faster than the old Mac Pro G5 2ghtz machine I used to use all day long when I first started using SU. The issue is less the power and ability but the marketability of an app and the real opportunity to make money off an app. Look how many really good 3D apps exist for PCs, not many really compared to many other software areas, when there is volume there are going to be solutions.
The other issue is interface. If you have tried Alias's Sketchbook Pro, it really has a great interface which works well with a tablet, less so on a PC unless you use a Wacom display. There may not be a great tablet 3D modeler now, one year after tablets finally hitting the mainstream, but give it another year with a wide variety of units available and there will be.
I will be picking up a tablet soon to be able to quickly go through a presentation or set of working drawings on site, being able to access, display and send something immediately without having to wait or carry more than I need is worthwhile for me. The more so when I travel and can only really do email and minor work anyway...carrying a pound of instant on equipment beats the 8+ pounds of laptop and ancillary gear a laptop requires.
Tablets are not ready yet, but it won't be long.