Ever the King of the zombie thread, I thought I'd resurrect another one.
I was looking at LEDs for the reverse lamps too and, having seen different bulbs recommended by different websites, decided to remove the bulbs from my old tail lights to see what they are.
Here's my picture and, although there are now some LED versions showing online, the light output doesn't look great. The best reverse lamps are around 1000 lumens and the only ones I could find were about 600 lumens. Not only that, but they didn't seem to have the connectors in the right place.
Here's a picture of the standard bulb in case anyone's interested.
Here's one of the bulbs I found:
http://www.ebay.com/itm/25W-CREE-PH1...-/130968808051
Hope this helps someone. I'll post if I do manage to find any direct replacements, but I'm not holding my breath.