1) Yes 3G speed is theoretically capable of streaming video. However, as always, depends on available bandwidth where you are.
2) Actual capabilities depend on the App and whether your carrier has approved those services. For example, at launch the ABC TV player did not work w/ 3G, but was later fixed. Skype now works w/ 3G but will be a paygo service on 3G in a few months. The Sling Player is anticipated soon, but no word whether it will stream over 3G. The iPhone app does, so I assume the iPad app will too, but since it will use more bandwidth its not a lock.