As of today, Thanko are taking reservations for their USB Wii Remote Chargers to be released at the end of this month. Priced at 3,480 yen each (or 6,480 yen for two), these plug into the back of your Wii and, you guessed it, charge your Wii Remote (along with two bespoke AA lithium ion batteries included with the charger). It takes about 3 and a half hours to charge the batteries, which then last for a good 25 hours.
To be honest, it's amazing that Nintendo didn't include this kind of functionality with the Wii in the first place.