I agree that it can be done on a road bike.

I live on an uphill 1/4mi dirt driveway (about 8%). Also, I live in hills above my mountain town, and have to ride about 600ft of steady uphill at the end of every ride. There's a dirt road that I sometimes take that saves me about four miles on the way home and I ride it on 23s every time. Once, I even rode that way to get home in the dark (forgot my light). Anyway, there are times when I'm out riding and I see some singletrack and decide to make a detour, and I've been known to ride bmx tracks (trails?) including the jumps!

Anyway, my advice echoes merlin's. Get lower gearing if you can, and watch your weight distribution. Also, watch out for soft sand and pea gravel (like the kind they pour four inches thick for driveways!). Those are the things I find the most dangerous on dirt roads. Also, if you don't already do this, always brake in a straight line and learn when to use each brake.