The USEF keeps the official rule book for USDF Dressage shows. They've opened things up to ANY noseband, include figure 8 or gackle nose bands. You just can't use a noseband with spikes on the inside (OUCH! Didn't know they made those...). Bits have gotten very liberal as well. You just can't use any gags, bits with "hooks" (like some Mylers), or bits with a middle piece that revolves.
I have the bit part of the rule book saved on my computer. I'll attach it below.
For the full rule book and to check nose bands, go to 2010 USEF Rulebook
And click on "Dressage".