jkOnTheRun notes that lots of people are experiencing data connection problems with T-Mobile and theorizes that the possible reason is that T-Mobile has finally gotten wise to the "T-Zones Loophole", which allows you to get access to the full internet while only paying for the supposedly WAP-ony T-Zones service. A photo of an internal T-Mobile document backs this up.

It's intermittent (I haven't experienced it yet) and not limited to T-Zones users. Have you had any hassles lately with data plans?

Some folks are offering the theory that T-Mobile finally caught on to users who have been using the web services by exploiting a hole in T-Mobile’s setup and that the carrier has blocked all the various ports to shut that down.  That is backed up by this internal T-Mobile Customer Service screen (posted on Howard Forums) to help reps deal with calls about this issue:

Read: jkOnTheRun: T-Mobile data plans suddenly stop working