I had the same thing happen when I moved, after I racked up around 800MB of data on my tethering (easier than you think). I didn't get a text, but I'd get redirected to an "upgrade now!" page. I spoofed my user agent on my laptop for the next couple days (because, shit, I'm paying for it, I'm going to use it), and had to do the same when I tethered for about a month or it'd redirect me. Since then, nothing.
I've noticed that it redirects when I use a largish amount of data (100mb?) with a desktop useragent. It redirects me to the "upgrade now" page for the rest of the month.
