fermion writes "If seems that Google's Android and T-Mobile have not learned from the bad experience and wrath Apple incurred with roaming charges on the iPhone. Applications can switch to roaming and data operation without the user's knowledge. Also, according to The Register, there is no way to switch off roaming. Given the backlash that Apple experienced over international roaming charges, one would think that T-Mobile would have built a phone to prevent such unexpected charges."
From the wording of the article, the inability to turn off roaming seems to be on a per-application basis; users can evidently disable it globally.