I have been looking to switch to OAuth for my Twitter integration code and now that there is a deadline in less than 7 weeks (see countdown link) it is even more important
I recently had to implement "post to Twitter".
What I found was that none of the various projects would work out-of-the-box. They're all close, but not quite there.
Eventually I settled on OAuthConsumer, Stig Brautaset's SBJSon, with some ideas more than code borrowed from Ben Gottlieb's Twitter-OAuth-iPhone for pin processing.
I probably would have looked at MGTwitterEngine, but I didn't need to hit that much of the API - just status updates.
I seem to recall having to hack OAuthConsumer to set a few parameters correctly - that was the biggest stumbling block.