Whilst Apple's approach (of trying to access http://www.apple.com) is a neat one, they presuppose that the page you're taken to is some kind of form entry page which has a single login button. Neither of these assumptions hold for the T-Mobile hotspot, and so the only thing you can do is press Cancel. However, if you do that - you're booted off the hotspot and WiFi connection is dropped.
Whilst the approach is a good one, it's clear this hasn't had much testing. For example, what happens if the password is wrong? Or what happens if there's a two-page process that you need to fill in? Due to the secrecy behind the development of the firmware, it's clear that none of these edge cases has been considered.
Which is a shame, really, because the idea is a good one. Now, if only it worked ...