Update: We just had a chance to get a quick hands-on with the little guy, and well, it's a battery charger. Apple tells us the charger has the lowest vampire power draw of any charger on the market -- the idea is for users to keep two batteries in each of their peripherals and two in the charger, so they can quickly swap out as the cells run out. Interestingly, the batteries are some of the only Apple products in recent memory that don't have an Apple logo on them -- they're just plain silver with "Rechargeable" printed on them. The charger itself has slightly nicer charging contacts than the usual spring-and-flap arrangement, and it features the same removable flippy-prong AC plug as Apple's laptop and iPad power adapters, so you can theoretically swap it for a longer power cord if you like.
Apple Battery Charger hands-on