You don't say wether you are "mountain biking" and want to take a camera with you or going to take pics on your MTB, so my suggestion is based primarely on the latter, though not entirely. I have done some fairly technical singletrack with a trailer and found it alot more do-able than you may think.

I used a BOB YAK, the suspended one. It is long enough to accomodate a reasonable sized tripod and is very stable with the weight a light-ish 4x5 set up would come in at. I had about 35lb on mine!!! The bike tracks way better than with panniers and I found I could tow at least 50% more than I could carry.

As for cameras I had a Tachi and found it very light, I tried a Crown but as I like uprights more than landscapes the fact that it had no rotating back a pain, no swing meant that it had no tilt when I had it on its side. If you shoot mainly Landscape format it may not be an issue.

Gari