Chris the reason that NASA claimed that sample return missions were safe was BOTH because it was believed the Martian surface was self-sterilizing and because it was claimed the return capsule could be made virtually impossible to failure.
You are aware that when scientists claim the Martian surface IS self-sterilizing they are saying there is no life near surface right?
And look at this article from Imre Friedmann, a noted biologist:

January 23, 2001
The Greening of the Red Planet
By: Henry Bortman
"Although Mars may once have been warm and wet, the red planet today is a frozen wasteland. Most scientists agree that it's highly unlikely that any living creature, even a microbe, could survive for long on the very surface of the planet."
http://nai.arc.nasa.gov/news_stories/news_detail.cfm?ID=211

And Friedmann is a member of NASA's Astrobiology institute!

Then look at this article by Chris McKay one of the most well-known Mars scientists:

"Bringing Mars to Life":
A transcript of the March 31, 1999 public lecture by Chris McKay
"We always thought that Mars was the planet that was most likely to give us information about life and which is why in '76, years ago Viking landed on Mars to search for life. It had a robotic arm it took some dirt did some simple biology experiments and basically found that the surface is dead, "Its dead Jim!" was the main result from Viking. Why is Mars dead?
"Fundamentally the reason we think Mars is dead, the reason we are willing to extrapolate from this one little hole in the sand to the whole planet is that nowhere on Mars today do we see water in the liquid form. We see water as vapor, we see water as ice, but it's never as liquid. Fundamentally the reason is that the pressure is too low. Water on Mars behaves like dry ice does on Earth. And one lesson that we know about life on Earth is that it requires liquid water on a planet. No liquid water on a planet, no life, that’s the end of the story."
http://chapters.marssociety.org/toronto/Event-Archive/McKay.Transcript.1.shtml

Note that quite key to Levin's argument for life on Mars is that liquid water can in fact exist on Mars' surface at least for short periods. It is therefore quite notable that steadily increasing evidence supports Levin in this contention.

Bob Clark