Hi Quentin,
Most of our interfaces are set in Helvetica (Georgia for permalink page Tweet text), although I think the most recent Android client uses Roboto for system consistency, and the Windows 8 app uses Segue UI as per Windows 8 standards. The typeface we use alongside the Twitter bird sometimes is Gotham Narrow, which is also used on physical printed materials, I think. All other branding info can be found on http://twitter.com/logos.
Ben