The batteries, from Sanho in California, are available in 4 different capacities, 60Wh, 100Wh, 150Wh and 222Wh, giving a computer like the MacBook up to 32 hours of battery life. The company is also offering a car charger. These items are all available for pre-order and will ship later this month. Prices are $149.95US (car charger), $199.95 (60-watt-hour battery), $299.95 (100Wh), $399.95 (150Wh) and $499.95 (222Wh). The batteries also come with a USB power port so you can charge an iPhone at the same time or separately.
The first thing I thought of when I saw this announcement was how the company was able to offer a charger with a MagSafe adapter, because that design is protected by Apple patents. I checked with the company president, Daniel Chin, and he told me: "What we did is obtain the MagSafe connector and cable from the original MacBook AC adapter, remove the adapter and splice in our own charger plug, which interfaces with our battery and car charger products. Since we are using the original Apple MagSafe plug and did not modify the actual plug (which the patent in question covers) in any way, we are still respecting Apple's IP and in no way infringing upon their patent."
That might or might not satisfy Apple, but it did allow the company to offer these chargers which are likely to be highly desired by some customers. At the very least, I'm surprised Apple has not offered a car charger for laptops. I can think of many times such an adapter would have been handy.
Update: Several commenters have pointed out the QuickerTek and MikeGyver power solutions for the MBP line, which predate the Sanho offering and use the same approach of sacrificing a MagSafe adapter to provide the plug connector. Apple's Airline adaptor, while electrically similar to a car adapter, is not recommended for automotive use (some have tried it with success, your mileage may vary). Lastly, you can of course use a DC inverter for in-car charging, but please be careful not to obstruct vents or leave the device plugged in and unattended... they can get very hot while in use. Happy motoring!