I know these are not OEM, but earlier this year (Jan) I had picked up a pair of microUSB (car+travel) chargers from all4cellular for my EVO 4G. I am using those with the 3D - so far, they are working great!
2 in 1 Wall & Car Charger - $4.95 - All4Cellular.com we carry Batteries, Cases, Chargers, Headsets and Much More
Good thing is that when I travel, I just need to carry this one with me (for use in the car and at destination) - accepts up to 240v, so can be used overseas too.
It was $2.98 for 2 of them (plus $2 for shipping) - total $4.98 (this is in Jan 2011 - looking at my receipt in the Email)
Looks like the price has now gone up to $4.98 each! (plus $2 shipping) - ouch.