Something that has not really been discussed yet is the results of the Viking Landers back in the 1970's. As people might recall, the initial results from the life experiments suggested that there might be life on Mars - when mixed with water and warmed, there was evidence of respiration. The problem was the equipment on the Viking was primative and weight constrained, so vital experiments stayed on the ground.
However, based on the soil chemisty there was an alternative, more plausible (at the time) explanation that the soil composition itself was leading to a chemical reaction which resemble respiration. After all, it looked like the planet was bone dry.
However, one of the requirements of the alternative explanation was that the soil was of a particular composition. Obviously, we're not back at the VIking landing sites, but where the current rovers have landed they are finding a salty soil, with indications that these are, in fact, brines.
The chemistry needed for the Viking results is apparently highly unlikely if there is a brine.
So. Did the Viking Landers discover life?
Hard to say at the moment. The Rovers can't detect life and they are in different locations, so the soil conditions at the Rover sites might be very very different to the two Rovers. There is a more fully featured, larger Rover planned for 2006/7 - this will have a greater range and life detection equipment.
If we're not alone... and I suspect that where there is water, there is life of some kind, then will that make people want to go all the more?