If you install the T-mobile app which lets you know your monthly usage, it claims I have 100 megabytes of tethering data. I rather quickly uninstalled it since it's a pain, but some limited tethering does seem to work for me -- I've only used it occasionally to SSH/mosh, but after a while it does an http redirect to a "No tethering for you!" screen; I assume after that 100mb cap is reached (though I haven't checked)