As the RadMediaPlayer has embedding the Silverlight media element it supports all the media formats that are supported by the SL platform. More about supported media formats you can find in Supported formats in SL2 and Supported formats in SL3-beta articles. Even in the mentioned articles there is no information about the MP4 format, Microsoft claims to support H.264 in SL3. Some more information about SL3 and MP4 you can find in following articles/presentations:
Regarding this information we think that official SL3 will support MP4 if packed streams are H.264 or AAC.
Attached you can find an example built with RadMediaPlayer for SL3.
the Telerik team