FWIW, T-Mobile has recently withdrawn this plan, it is no longer available for new or switching customers. Cheapest "unlimited*" prepaid data plan is now $50 a month.
Oh! Sorry, looks like I'm wrong. I do see it being offered on their website, but it's for new activations only - that would explain why I was under the impression it's no longer offered after not seeing it in the prepaid plan switcher in my account.
