I think the thumb keyboard is pretty good. I'd love to see some slight mods to it though:
1) The virtual keyboard shoud be a transparrent overlay over whatever you are busy with. Then you would be able to, for instance, follow a conversation in GAIM while type.
2) The ENTER key seems to do a CTRL-LF instead of an enter. So typing in Skype is a bit of a nuisance - one has to type, then leave the thumb board, then pull up the stylus board again to click enter and send the text. A pain. (Or maybe I'm doing it wrong)
3) I found that in some apps (I think notepad/Maemopad+/browser ) ot sometimes gets confused and starts duplication long paragraphs.
4) The predictive text is fair, but could be improved in this way: When typing a long word, say 'communication' - by the time I have typed 'comm' it predicts 'community'. Now this is not the word I want, so I keep typing. After another keystroke I have 'commu'. The N800 should now realise that I don't want community, and try something else, but it keeps proposing community until I get to communic, at which time it switches - a full three keystrokes late. Or is ther maybe a different way to do this with gestures (eg) that will flip through various suggestions?