Yes it does.

Yes it does.

Yes it does, for one year. You can extend the warranty to two years with the AppleCare Protection Plan for iPhone, for $69. 

iPhone batteries are not user replaceable. If your battery stops working while still under warranty, you can bring the iPhone to an Apple Retail Store or to an Apple Authorized Service Provider, and they will take care of replacing the battery (not immediately though, it may require some days). For out of warranty batteries, Apple made available a Battery Replacement Program, which costs $79 + 6.95 shipping.

iPodJuice is selling iPhone batteries directly to the customers, but you must be familiar with soldering to replace it by yourself, since the battery is soldered to the motherboard.

The official specifications say the following:

• Up to 8 hours talk time
• Up to 250 hours standby
• Up to 6 hours internet use
• Up to 7 hours video playback
• Up to 24 hours audio playback

The actual battery life will of course change according to your mix of wi-fi, iPod and phone functions. Our suggestion is charging (or synching) the iPhone overnight.