How do I sign out of a Twitter account on the iPhone?
The only way to sign out of an account is to actually delete it, according to the UI. This is tied into the Twitter integration that was part of iOS 5. 

To sign out of the account, you need to:


Go to Settings
Choose Twitter
Tap on the account you want to sign out of.
Tap Delete Account


This does not delete your actual Twitter account, it just logs you out and removes that accounts credentials from your device.