Recently Twitter released xAuth using which applications can request user tokens by just providing their username and password. This has been a pending request for a long time mainly by desktop and mobile based twitter clients. For them access to the web may not be straight forward and it was difficult to get oauth access tokens.
But the access to xAuth is limited. You need to send an email to firstname.lastname@example.org to enable your application for xAuth. Once they have enabled your application, you can pretty much follow their documentation to get it working. Documentation is far from clear right now, and it may take a lot of iterations and exchange of emails to twitter to get it to work for you. As was the case for me. But finally I got xAuth working for SMSTweets, and I have moved all my users who had provided their username and password over mobile phone to OAuth based authentication.
And yes, Now I can get rid of all the duck taping that I had done to get OAuth based and HTTP based authentication to work.