I love that Twitter is baked directly into iOS.... but could be so much more - and I've written about that from the launch (example here). My primary frustration is that the integration doesn't do enough with my contact list - which remains the most accurate, important social network. Furthermore, Twitter uses aliases - and I rarely remember people's Twitter usernames. This problem exists even within the Twitter app or popular readers like Tweetbot. Apple has a Twitter Contact tool - but it doesn't do enough: I should be able to connect with the user by name and not by username. After all, my contact list is the more natural convention and, of course, how I know these people.
In the below screenshot, I Tweeted a TechCrunch article by Anthony Ha - and the only reason the association worked is because his Twitter handle is his name!
Big opportunity for Apple to make Contacts more social and usable. And for Twitter to grow usage and cement user identity.