I used a WAN card (a csu/dsu) with Linux. It was a card that was not supported by the Linux kernel, so I had to obtain the driver source for the card and compile the drivers. The problem I had with that card was that I could not compile the drivers with any kernel newer than 2.0.29. The card was a RISCom n2csu/dsu and I used it successfully with frame relay.
-- Andrew
