Just to throw in my two cents, I love Millenicom. I have the Franklin CDU-680 and it's the perfect setup - no download limits, no contract, and $49 a month. You pay a $50 setup fee but that includes the modem. If you lose it, you gotta pay $200 to get a new one though, so hold on to it!
Of course, when
Sprint Xohm Moible wimax comes out, I'll be switching to that...it's much faster and will have no contracts. There will be discounts for signing up for extended periods though.