There's been some past discussion of Mac compatibility with Sierra Wireless broadband PC cards, but there's a new speed leader: Verizon's BroadbandAccess plan, with sustained transfer speeds up to 500Kbps and bursts up to 1.5Mbit (at least if you dwell in a coverage area). $80 a month for unlimited access is the price of entry.
I had the opportunity to try one of the Sierra Wireless PC5220 cards in my PowerBook at the Verizon launch breakfast last week at the Waldorf=Astoria (yes, it has an '=' in its name) in NYC. Although you won't find mention of Mac compatibility in the Flash manual for the card, and though Verizon and Sierra Wireless reps were unaware of it, there are drivers for the card built-in to Mac OS X as of 10.3.5 (and available from Apple for 10.3.4). Just pop in the card, and a new network interface shows up automagically ... walk and surf at the same time!
Mac OS X Hints
http://hints.macworld.com/article.php?story=20041101095139926