I recintly seen this project that has some support for USB modems you could have alook and see if they support it. but most people on hear get it through ethernet as I do they have suppot for my adsl modem sent to me from AOL [UK]
have alook hear I have used linux for 3 years on and off and still not got it working yet only tryed this once have alook though you dont need any changes to the kernel of kernel modules so it looks easy
http://eciadsl.flashtux.org/tutorial.php
agen have alook through the back end of the website for informaton
I am sill glad i bought my rooter 100%
rember that wifi adaptors are not supported or are just to flaky
some are though! just dont risk it with out some rechearch first
I just tryed to get it working to build up my linux knollege
before that i never botherd
it should work with all ISP's - do buy one from your contry though