http://wipcore.t-mobile.com appeared on my safari homepage.
i rebooted 5 times and nothing worked so i called tmobile. they know i have an unlocked iphone and they know i was only paying 5 bucks a month for internet and i explained this to them and they explained to me that they're updating their systems to register the imei number of ALL THE SMART PHONES THEY CURRENTLY ARE GIVING SERVICE TO. then all of these iemi numbers will be on their poop-list, so that internet access will not be provided to these unlocked phones anymore if you pay the minimal $5.99 or $9.99 fee. they said that slowly but surely all of these phones would lose "free" internet. so, i demanded a refund. they gave me a $45 credit for the internet service that i've "paid for" over the last 6 months with them.
hopefully a work around will come, but i imagine if you re-register a different phone on their site and then put the sim back in the iphone it might work for a while.
if there ends up being no work around, call tmobile and demand compensation. they did it for me, and they'll do it for u. if they get enough calls maybe they'll restore things to how it was yesterday.