Client-side validation (checking for things like required fields, valid email address, etc) is always one of those sort of pain in the butt, last minute, only because i have to sort of things. It's not usually fun or flashy, it's just one of those things you have to do. Enter VanadiumJS, a new library just released by Daniel Kwiecinski.
Speaking of HTML5 and
canvas, Carsonified has a nice little tutorial to get started with
canvas. This article includes a good overview of where you can use
canvas(anywhere), which browsers support it (most of them, at least with a little help (even IE6!)), and how to get going with some easy stuff.
There are no good desktop Twitter apps out there. I seriously can't stand any of them. So this is a plea to the software developers out there: Please, make a good one. To get you started, here's a list of features I'd like to see: