Recently I have not been able to use my Twitter iPhone app. due to getting the message:
Your username and password could not be verified. Double check that you entered them correctly and try again.
Google didn't really help me then I got an iCal reminder from an event in August and it dawned on my that my date/time wasn't set correctly. As it turns out my phone thought it was August 28th 2010 today (really its October 24th 2010). A quick change and it all works fine. It would of been nice if the error descriptions was a little bit more detailed but its working now and hopefully this helps someone else.
n.b. I am aware as well about getting the iPhone to set the date & time automatically which I thought I had - subsequently its set now