As promised, here are the details for making your own, I'll-only-pay-for-this-once "portable Internet." The goal here is simple: we are going to take a standard wireless router and use the "antennna-side" to connect to an existing wireless network as a client, and then use the "wired ports-side" on that hub to connect our machine(s) to the Internet.
This is really a trivial exercise. There are only two things you'll need:
1) A wireless base station that supports dd-wrt;
2) A copy of dd-wrt.
The first is strictly a matter of preference and budget. I went with the readily available Linksys WRT54G model. You can find them all over the place, but whatever you do, make certain that you look at the serial number prior to buying it. The newest releases are not able to run any version of dd-wrt, as Linksys (in their infinite wisdom) has somewhat crippled them by reducing the amount of flash memory they have. Ensure that you have nothing later than version 7.0 by comparing the serial number on the outside of the box against this chart on Wikipedia.
If you want to spend a bit more, but get a machine with some additional horsepower, then the kind folks at dd-wrt recommend the Buffalo Airstation WHR-HP-G54. It costs a few dollars more, but it's worth it. Not only is it more aesthetically pleasing, but it also has loads of memory, and (so I am told) a better range than the Linksys models. I didn't do this. I wish I had.
Once you have your wireless router, simply download the appropriate version of dd-wrt by going here. Installation is trivial; just follow the instructions here.
Once you have that done, this is a trivial exercise.
1) Run an ethernet cable from one of the router ports on the back of your wireless hub to the ethernet port of your pc/mac/laptop/whatever.
2) Start up a web browser on your computer.
3) Access the following URL http://192.168.1.1
4) Click on the "Status" tab. When prompted, type in the username/password to get into the admin tool (by default, these are set to root/admin. Please change them). Then click on the "Wireless" subtab.
5) Scroll down till you see "Wireless Nodes". Click on "Site Survey".
7) A window opens showing all available networks. Pick one that you can legally connect to, and click "Join". When a button allowing you to continue shows up, click it.
8) Next you are bounced to the "Basic Settings" for wireless config page. Ensure that "Client" is chosen in the drop down menu, and click "Save Settings".
You are done. Wasn't that easy? For future connects, you simply have to repeat steps 3 through 8 to pick the network you want to connect to. It only takes a few seconds. Personally, I find performing a 10 second routine is preferable to giving someone $50/month.... but that's just my opinion.