This is the same issue I’ve posted on a couple of forums. It’s seemed detailed enough to be a blog post, so here it is. Under 10.5.8 (Leopard) I was successfully using my Merlin XU870 ExpressCard (supplied by ISP) with Three Broadband (3) in Australia. I did not use the 3 drivers, but used the Leopard WWAN built-in to connect. There was never an issue as Leopard detected the card as a “Novatel Wireless HSDPA Modem”, and I configured the Network Advanced settings for 3 postpaid services as: