I understand it answers the question "What are you doing"... But do you have to sign up for twitter accounts at twitter.com to use the app store apps?
Also, what are some good feeds to watch/participate in?
Thanks =)
Yes- all of the iPhone apps (and clients for other systems as well) rely upon your having an account on Twitter.com. Well, OK, that's not
completely true, there are some other Twitter-ish alternatives that some of the apps use, but by and large you need an account on Twitter.com.
As far as finding good feeds to watch/participate in, I by and large just have my friends, a few celebrities who post interesting stuff, and BBC news. You might try grabbing one of the apps that has popular feeds included in it by default. You might try out the free version of Twittelator- it has a 'featured tweeters' page in it that might be of interest...