I've been playing with CoTweet lately, now you mention it – lots of potential there for nonprofits who have staff or volunteers taking it in shifts to man their Twitter account, or for Twitter users who have to juggle both a personal and an organization's account. There are a few usability issues that send me over to the Twitter website accidentally with the odd unwary click, but on balance I think it's a real timesaver, even beyond the ability to schedule updates.