Since some folks geek out on this sort of thing:
I’ve been using Twitter exclusively on the iPad this week. It has the advantage that I can’t be distracted by it while I’m working on other things, and the screen is big enough to be comfortable viewing links and pictures. I clip any interesting articles to Instapaper unless I have time to read them right now, which is an improvement over my previous tendency to leave them open in yet another Firefox tab which just gets in the way later. When the iPad gets threaded email this fall, I might see if I can handle mail the same way (Gmail in the iPad browser is nice, but I have more than one account to keep track of).
I’m using Tumblr as a kind of public notebook. Altered photos and screengrabs, snippets of commentary or rant, quotes from something I’m reading. For notes I don’t want to share, I’m using Evernote. Still getting the hang of what sorts of folders are useful, and trying to remind myself to move iPhone photos into there when relevant.
My todo list still lives in The Hit List even though development appears to be completely stalled. I like the interface and how it handles recurring items too much to switch unless forced. Most of the time it doesn’t matter that I can’t sync it with my phone—the only reminder I seem to miss is a result is my twice-a-month contact lens switch, and being a day late isn’t terribly fatal. If I really need to track a errand or grocery list while out and about, I put it in Evernote.
One of my consistent goals with all of this is to have a system that’s so calm and easy to maintain I don’t think about it. When I’m working, I want to minimize distractions and have everything I need easy to access. When I’m reading or browsing information, I want it to be so easy to clip and track I can find the good parts again later. So on and so forth.