Apple's just announced that it's bringing deep Twitter integration to all of its iOS-based devices, and to many of its own apps, including Camera, Photos, Safari and Maps. That integration also, of course, extends to Contacts, where you'll be able to link your contacts to their Twitter handle and keep their information updated accordingly, much like Android. You'll also only have to sign into Twitter once (in Settings), and then simply share those credentials with any app that requests them.
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iOS 5 Twitter integration