I'm typing this message on Dell Inspiron 1300 running OSx86 10.4.6 with Sierra Wireless AirCard 595U and Sierra Wireless Watcher Lite. The Sierra Wireless AirCard 595U will work with any Mac running OS X 10.4 and above with free USB Connection.
So far, I've tested with Dell Inspiron 1300 and Dell Optiplex GX-260 both running OSx86 10.4.6. The software, Sierra Wireless Watcher Lite, is PowerPC only but it works just as well after getting started. The device works flawlessly and offers download speeds of up to 500K over Sprint EVDO Network (!)
I'm wondering if anyone else had similar experience with USB Cellular Cards. By the way, our company purchased the Sierra Wireless AirCard 595U to run with Windows laptops. The fact that it also supports OS X is a bonus (!)