M.C.
OMG! Isn't that ridiculous! My experience wasn't quite as severe, but I can confirm that I was told the same thing about retroactivating support amounts. Our support was set in our decree at $125/mo. with a 5% increase yearly for cost of living expenses. Yeah right! That increase didn't factor in ANYWHERE on the State's registry, so as the years passed and he didn't pay (didn't pay a dime for 10 years, yes it was terrible), his back support didn't accumulate nearly as high as it should have. After going back to court, our support was raised, but it took that magical piece of paper to make things happen.
The good news is (at least it was for me) is that eventhough our decree says that support was to stop when our daughter turned 18 if she didn't go straight to college, that since he owed a little bit of back support still, the collecting of support will continue until she's 21. Nice for her (now age 20) since she had to do without so many extras when she was younger (she's buying her own car with the support - nice justification since we always drove an embarrassing heap when she was young!).
My only advice would be to allow the State to collect and distribute the support if that is an option. It was nice not having to deal with him about the money - it kept our conversations focussed on less tense things. Also, the state puts the money on a local bank debit card so it is easy to access AND they'll go after him if he doesn't pay.
Good luck.