David, an employee of Thumbplay, replied to Why won't the app work over 3G on Sprint HTC EVO 4G?, a question about Thumbplay.
Update (2/10/11): The following is from a Sprint representative. Sprint can indeed fix the issue. Please take note of the Toll Free number 1-888-211-4727 and key words (in bold) that can be referenced to help the rep identify the issue/fix.
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As many of you have reported, there was an issue identified shortly after the launch of the 3.70.651.1 maintenance release that caused a problem with streaming applications. The root cause has been identified and will be fixed in the next maintenance release.
In the interim, there is a workaround that can be applied by Technical Support. As of 2/1, an updated workaround is available that corrects both HTTP and RTSP streaming (the previous workaround only corrected the HTTP streaming). /Please contact Technical Support at 1-888-211-4727.
Although changing the default proxy settings will resolve the issue temporarily, these settings are not the issue. If you have changed the settings as a workaround, you may need to reset them back to the default settings prior to the next maintenance release.
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Update:
There is no case/problem number for this issue. It is classified as a known issue in Sprint's device portal and listed as 'HTC EVO 4G (A9292) - UPDATED 2-1-2011: Streaming multimedia applications receiving errors on software version 3.70.651.1'. The rep will use an internal tool to apply the workaround.
The new PRL is 60675.