...you may no longer have a working card. If you upgraded to Apple's 10.4.10, you may have noticed that your card no longer connects. This happened to me last week. I'm told Apple has created a WWAN update, but I've also heard that it doesn't fix the problem. Anyway, if you want to know how to fix it, go here. Just follow the directions. Trust me: they work.
By the way, if you don't have an HSPDA card, I'd strongly encourage you to get one. Well, only if you travel a lot. At $60 a month, it only makes financial sense if you travel enough that you spend about $60 a month on Wi-Fi. I was finding that I'd log into a hotspot at the airport, and then at my hotel, and then at the airport again, and then at the next hotel, and then again at the airport. The bill was $50, and that was just one trip.
I use a Novatel Merlin XU870 with my MacBook Pro, and it's heaven. Aside from the temporary glitch with Apple's OS X update, it "just works." Not super fast, but fast enough to crank out blog entries from the back of a taxi.