FWIW I wrote the above
FWIW I wrote the above drivel. I am a reporter who has built several electric vehicles and knows a thing or two about they work.
The battery cells are made by Panasonic, that is a well known factoid. I believe it is discussed in Tesla's quarterly filings with the SEC, for example. A123 was never in the running to make Tesla's battery cells. The battery pack design and battery management system are Tesla's own technology.
When they tell you "battery life" the sales person is going to be giving a simplified answer. There are a lot of variables to actual battery life, such as the number of charge cycles, the number of times the pack is quick charged, the ambient temperatures, whether you're a lead-foot or not, etc etc etc ... read up on the angst Nissan Leaf owners have because of uncertainty over battery pack capacity loss, and the simplistic information Nissan is telling the public. There are a lot of technical details, however, that I imagine would be very difficult for an automaker to explain to the public.
How did someone get 400 miles range? First, when Tesla's sales person quoted you "300 miles" that's Tesla's number rather than the EPA certification. Second, how an individual would do it is by driving in an efficient manner - such as sslloowwllyy. It's a simple matter of consuming fewer watt-hours per mile, and presto whizzo you get a longer drive. The important measure is the kilowatt-hour capacity of the car. With a vehicle like the Model S it'd be awfully difficult to keep your speed that slow.
As for written guarantee on a replacement battery - unsure what exactly you're looking for. However, the company did recently announce a replacement price for the battery pack. I don't know how iron-clad that is, however. I'd imagine it would be foolish to announce an iron-clad price for a product you'll be buying in 8 years time.