Isopods are very commonly raised as feeders by people who keep salamanders, turtles, frogs, and other small lizards, so it stands to reason they wouldn't harm bearded dragons. Look around on a site such as caudata.org (the salamander site) and you will find them referred to quite frequently. Amongst those folks, raising isopods is as common as raising superworms or roaches is among the bearded dragon enthusiasts.
Why they aren't used by bearded breeders, I couldn't say. My best guesses would be it's either their limited size (they wouldn't be much use for adult dragons) or maybe bearded dragons just don't like the way they taste (Melissa Kaplan mentions on her site that her dragons don't like them, but that other people she knows feed them sucessfully).
Here are a couple other sites that make reference specifically to feeding them to beardeds:
So, I will say again, they are safe to feed, but your dragon may not like them. According to the experts, they are very high in Calcium and very easy to gut load. Because of their shells, they shouldn't be used as a staple but they are fine to feed on an ocassional basis if your dragon will eat them.
I have to through my 2 cents in. Bearded dragons in the wild never get so spoiled that they can chose what to eat. However my little firecracker eats Rollie Pollies all the time, I grab her 10 or 20 when I see them and she chows them down. Two weeks ago she snagged two wasps right out of the air(she's fast). I wasn't to thrilled about that, but she was fine.