T-Mobile G1 users be warned: some third-party applications will continue to run in the background, even if you have turned your data roaming settings off. T-Mobile USA issued a statement today (picked up by Engadget) that said customers could get slammed with expensive data roaming charges if they’re not careful to keep those applications closed while traveling. T-Mobile said: “Some third-party applications available for download on Android Market require access to the Internet and have the ability to override your data settings when in use.” Customers are warned whether an application will use the Internet before its downloaded, however, they aren’t explicitly told about the roaming implications. The carrier said data roaming can be shut off under the phone’s settings, but that doesn’t go far enough—third-party applications should be kept close during the trip. Clearly, this is an issue with the Android operating system. But it’s not clear if it is a flaw or if it was intentionally built this way. For instance, perhaps you’ll want to use selective applications while traveling, like maps for directions, without turning on all data capabilities?