What I use for mobile broadband is Virgin Mobile broadband to go, they just revamped their plans and for $40 month you get unlimited time/data and no contract (woooo).  You can either get a USB style 'dongle' (heh) for around $75 or if you really want to get fancy they have a portable hotspot (or mifi) for $129, with that one you can share your connection with others (if you want).

I use mine mainly for business so I can connect up to our networks when a server cries for help or a user needs some handholding :)  and it's worked quite nicely, speeds aren't as fast as home but for what I need it's great.  My wife has also 'borrowed' it from time to time so she can do stuff on her netbook at lunch...just wish she'd tell me when she does so I don't go crazy thinking I left it someplace.... (sigh)