1. A computer-readable memory storing computer-executable instructions for causing a computer system when programmed thereby to perform: selectively enabling temporal motion vector (“MV”) prediction for deriving MV information for blocks of a current frame in a sequence of video frames; encoding the blocks of the current frame, including, for a given block of the blocks of the current frame, encoding the given block of the current frame using motion compensation with derived MV information relative to a reference frame to determine a motion-compensated prediction for the given block, wherein: if temporal MV prediction is used for the given block of the current frame, the derived MV information for the given block of the current frame is derived by performing temporal MV prediction based upon MV information of a block of another frame in the sequence of video frames; and if spatial MV prediction is used for the given block of the current frame, (1) the derived MV information for the given block of the current frame is derived by performing spatial MV prediction based upon MV information for one or more surrounding blocks of the current frame, and (2) the reference frame for the given block of the current frame is selected from among reference frames used for the one or more surrounding blocks of the current frame; and wherein at least one of the blocks of the current frame is encoded using motion compensation with derived MV information derived using spatial MV prediction; and outputting encoded data for the blocks of the current frame, including signaling information in the encoded data that indicates whether temporal MV prediction is enabled for the deriving the MV information for the blocks of the current frame.