3G USB devices require drivers, and can be OS specific. 3G cards often contain their own drivers and utilities but often require specific OS support.
I think its fair to assume if youre using a a 3G USB card, your network port is not being used. A 3G NIC card with an RJ45 connector frees up the USB port. If set up properly, it could have a simple DHCP server, router, built in. Management of it could be via an embedded webserver. It would appear to the computer as an IP router, rather than a modem. It would therefore be platform agnostic.
I would suspect CPU usage would be lower, as its not on the USB bus, maybe incorporating a TCP/IP offload engine could help. I have no idea of the electronics involved but looking at a modern NIC there doesnt seem to be much too it.
(edit - power.. i forgot power. Could/do a laptop incorporate POE ports? Is the tradeoff the reason why USB is used?)-- dja, Jan 08 2009 CradlePoint CTR-500 Mobile Broadband Router http://cradlepoint....le-broadband-routerAlso does WiFi, and they have other models too. [krelnik, Jan 08 2009] Somewhat baked by CradlePoint's products, though they don't actually provide the 3G part themselves. You plug Ethernet into one side, a 3G modem into the other and boom. See link.-- krelnik, Jan 08 2009 One could instead easy-bake the same thing themselves with an old laptop, for a whole lot less.-- Spacecoyote, Jan 08 2009 A good idea, but in my opinion could be a little bit more sophisticated...Instead of RJ45, why not add 802.11 (WIFI) to those 3G cards?-- alexandre, Jan 10 2009 random, halfbakery