I spent ages with the oAuth code samples getting an app to send a simple tweet before I realised that there is an extremely simple way of doing, with no need to register your app with Twitter or use access keys. Use Twitter Web Intents (see the developer pages on the Twitter website for more details).
Just one line of code to send a tweet ...
native.showWebPopup(0, 0, 320, 480, "http://twitter.com/intent/tweet?text=My%20tweet%20goes%20here")
As well as the text parameter you can also use
The text and url parameters should be URL encoded. The user signs in to their twitter account in the web popup which then shows them the message (which can be edited) and the Tweet button.
You'll want to make the web popup less than full-screen to allow for a button to close the popup and return to your app. The user doesn't have to log in next time you call the popup so it's great for quickly posting scores etc. during a game.
Simple as that!