The manual for the Sony HDR CX-12 says you should not use a card reader which requires an adapter for the MS PRO DUO cards.
What are some recommendations for card readers for this type of card?

FWIW, I've had really mixed luck using readers for cards larger than 4G, sometimes they work, sometimes they don't, and sometimes it varies from computer to computer, making it even more "interesting".

Your best bet is to use the dock that comes with the cam, never had a problem with any genuine MS Duo using that approach.

I have a "SanDisk MobileMate MS+". It's specifically for MS Pro Duo cards, about the size of a thumb drive, very inexpensive, and seems to work fine on both of my editing systems (XP and Vista 64)- Oh, and also works on my Vista 32 laptop.