I was delighted to see this post in the forum and sorry that no one seems to have figured it out for certain. I've had this problem for a very long time and assumed it was a lack of balance but I haven't figured out what it is, yet.
I've tried diatomaceous earth but it doesn't seem to phase them. Likewise, letting the soil dry out between waterings barely slows them down. If I were to let the soil dry out enough to truly get rid of them, I'd also be rid of all my plants. (I've actually let plants die to see if it helped -- as soon as I started watering again, they were back).
I've noticed this seems to be a local problem -- I'm in the Greenville, SC area and I've talked to many gardeners here who have a difficulty with pillbugs. The community garden I help with is also dealing with them, as well as some other major gardens. I've wondered if there is something in our specific soil or climate that causes their populations to spiral out of control. Alternately, I've wondered if there are localized diet preferences for them? When I talk to people in nearby towns or even neighboring states about it, they look at me like I'm nuts. (Incidentally, I do know for certain it is pillbugs doing the damage).
I hugelkultur and compost, so I'm not sure why they don't have enough rotting organic matter and need to supplement their diet with other things. They also seem to favor some types of seedlings over others (and all root vegetables, especially potatoes). They don't tend to bother mature plants unless they can reach the fruit. I've found them pretty high up on tomato plants eating the fruit (along with earwigs, which are similar in their diets/behavior to pillbugs for me). I wonder if the earwigs are negatively affected by the same things the pillbugs are.
My chickens don't like pillbugs. They'll eat them at first but seem to tire of the constant, excessive supply. Can't really blame them.
Wish I had the "plague" that maikeru sumi-e mentioned -- sounds like it could be a cordyceps mushroom?
I think that next I'll try even more hugelkultur and adding a pond (which I wanted for water catchment reasons, anyway) to attract more frogs and toads.
If someone figures it out, please share!