"with the exception of complete lander/probe sterilization"

It is interesting to note that only landers with life detection instuments are required to be thoroughly sterilized as the twin Viking landers were in 1976. Because Mars Pathfinder was not a life sciences lander it was given a glorified alcohol swabbing and thus avoided the microbe killing Viking sterilization process. Since the twin MER's do not carry life detection instruments they too will be allowed to carry a certain bio-burden with them. Can terrestrial microbes susrvive in certain niches on Mars and have their spores carried in Martian wind/dust? Of course the Russians sent a few unsterilized probes to the Martian surface.

"Category IV missions comprise certain types of missions (mostly probe and lander) to a target body of chemical evolution and/or origin of life interest or for which scientific opinion provides a significant chance of contamination which could jeopardize future biological experiments. Requirements imposed include rather detailed documentation (more involved than Category III), including a bioassay to enumerate the bioburden, a probability of contamination analysis, an inventory of the bulk constituent organics and an increased number of implementing procedures. The implementing procedures required may include trajectory biasing, cleanrooms, bioload reduction, possible partial sterilization of the direct contact hardware and a bioshield for that hardware. Generally, the requirements and compliance are similar to Viking, with the exception of complete lander/probe sterilization. Category IV specifications for selected solar system bodies are set forth in the Appendix to this document. Solar system bodies considered to be classified as Category IV also are listed in the Appendix."

http://www.cosparhq.org/scistr/PPPPolicy.htm