This isn't very reassuring to me:

"Recommendation. If sample containment cannot be verified en route to Earth, the sample, and any spacecraft components that may have been exposed to the sample, should either be sterilized in space or not returned to Earth.
The engineering and design of any sample-return mission should incorporate some means of verifying sample containment during transit and prior to return to Earth. Means should also be available to sterilize the sample, and any spacecraft components that may have been exposed to it, in flight or to prevent their return to Earth in the event that containment cannot be verified.
Recommendation. Integrity of containment should be maintained through reentry of the spacecraft and transfer of the sample to an appropriate receiving facility.
The points in a mission where loss of containment is most likely to occur include operations on the martian surface; intervehicle transfer of sample material; vehicle reentry, descent, and landing; and subsequent transfer of the sample container to a receiving facility. Techniques and protocols that can ensure containment at these vulnerable points should be designed into the mission.
Recommendation. Controlled distribution of unsterilized materials returned from Mars should occur only if rigorous analyses determine that the materials do not contain a biological hazard. If any portion of the sample is removed from containment prior to completion of these analyses, it should first be sterilized.
Returned samples should be considered potentially hazardous until they have been reasonably demonstrated to be nonhazardous. Distribution of unsterilized sample material should occur only after rigorous physical, chemical, and biological analyses confirm that there is no indication of the presence of any exogenous biological entity. If any portion of the sample is removed from containment prior to this determination, it should first be sterilized. The development of effective sterilization techniques that preserve the value of treated material for other (nonbiological) types of scientific analysis should be the subject of research by NASA and by the science team associated with the sample-receiving facility.
Recommendation. The planetary protection measures adopted for the first Mars sample-return missions should not be relaxed for subsequent missions without thorough scientific review and concurrence by an appropriate independent body."
http://www.nas.edu/ssb/mrsrexec.html