I don't know about the MERs, but I read a long .pdf file describing the mission requirements for the 2009 lander, and it clearly said that mission operators/scientists had no proprietary rights to the collected data, and all data must be released as soon as feasible.

I suspect that the right to delay data release from the Hubble (and maybe other space telescopes?) is the exception, not the rule. Anyone know?

I suppose if the same release requirement holds for the MERs, they still could say they're working on the release as soon as possible, and are not required to release it immediately. They do have other things on their schedule.