Update: if you use OneNote 2003 with an Origami (Ultra-Mobile PC) device, you should get this hotfix which takes care of an incompatibility between the two (only get it is you find that ink strokes disappear after you write them). This hotfix does not apply to normal Tablet PCs - only the UMPCs that let you use your finger in addition to the stylus.
You may have heard some of the buzz about the Origami project - an ultra-mobile PC initiative that Microsoft is spearheading. You can learn more about it here. In summary it is a lightweight (<2lb) kind of Tablet PC with a touch screen which you can use with your fingernail or any pointed object. It has a 7" screen and no keyboard (although one can be attached).
I've had a bunch of questions about OneNote and whether it will ship on Origami devices. Bear in mind of course that Microsoft does not actually make these PCs. As with the tablet PC and other PCs, other companies make them so they decide what specific hardware and software go into the actual units. Microsoft just does a "reference" design to show what is possible and what we think would work well with the software.
That said, I expect to see OneNote 2003 (later 2007) on most of these devices because it is an ideal application for this form factor. You'll be able to take OneNote places you wouldn't bring your laptop, and record audio, take ink notes, draw diagrams, flag items to categorize them or for follow up and so on. With these devices you'll most likely be browsing, doing email, listening to music, and capturing notes.
OneNote 2007 has a new mode we currently call "minimal UI mode" (insert sexy name here). It's designed to cut out distractions for when you really just want to take notes. Those of you with tablet PCs also know that the more screen space that can be devoted to ink the better since you tend to write much larger than you type. OneNote 2003 also has a minimal UI mode, but it is only available when the OneNote window is very narrow (<300px or so) because it was designed for quickly jotting a one to two-line note or dragging something into it from the web. In OneNote 2007 this mode is available at all window sizes, even maximized. It hides the navigation UI (page, section and notebook tabs), as well as any extraneous toolbars or task panes and leaves you with just the set of commands you need to take notes on a page.
The Origami devices will have a 800x480px screen res minimum, but there has been talk of running them at 1024x600 or 1024x768 in software and using smart downsampling to 800x480 so that you seem to have more screen dots than are actually available. Either way, OneNote's new minimal UI mode is going to make the most of the dots you have. Here's a screen shot of OneNote 2007 at 800x480 in minimal UI mode.