Thank you all. To randomly answer some questions/suggestions: Johno02, you're right, the industry *is* changing; I could wait until it settles down but I'm not sure it's going to! And I'm not sure my modem will last that long. I may call Comcast or Verizon or whoever offers more land-based services (not sure what else to call them) just to see how they work and what they cost but if, as MrWizard says, they're all on contracts that's not a choice; I don't know how long I'm going to be here, it's all related to finances (or the lack thereof :) ), but I sure hope it's not another two years!
About Millenicom - I did try them out a few years ago and had a really negative experience. I don't remember the details now; some of it I think was about a lack of support for a modem they sent me that didn't work. It was bad enough that I'm not sure I want to try again.
The other carriers - I know about Virgin Mobile, T-Mobile, Verizon, Sprint, and Clear (which I think is now owned by Sprint). Virgin's and Clear's are within my budget but I've read mixed reviews about their service, everything from "it's wonderful" to misleading coverage claims and poor customer service, so if anyone here has used either of those I'd be curious about your own experience. My first cell phone was with Virgin; it was okay but it didn't get used much. When I dropped the land line at my last apartment I switched to Straight Talk because it was cheaper; I've been happy with them but unfortunately they don't offer modems too. I have heard good things about Verizon from other RVers but have always ruled it out for cost; I just checked again and the only thing I see that isn't a phone, a "Jetpack," is $90/month for 10 gigs. That's more than twice what I'm paying now and is out of reach of my extremely limited budget.
So - am I missing any other carriers? And have any of you tried any of the ones I've mentioned? I know I have a lot of research to do but this connection still seems slow so it will be a while before I can do all of that. Fortunately I have lots of offline projects to keep me busy...
Have a good weekend, everyone. I'll be in touch as often as I can.