These are very early results right now. Perchlorate is found in some types of rocket fuels.
Unfortunately, there's just not enough data to make perfect conclusions. The thrusters used to get to the surface used hydrazine, which is N2H4 (sorry for lack of subscripts).
I don't know enough about chemistry, but maybe some odd material in the soil reacted with it. I really don't know. It would have helped if Phoenix could have been a rover, which would have let it move away from the landing area.
Mars Science Lab should help out greatly, since it will have the ability to take samples, and it will be mobile. It will also be able to crush small rocks, enabling it to see what's inside - such things shouldn't be affected by rocket wash.
And we don't know how far down the perchlorate might go, or how much of the planet it might cover. All we can do right now is literally scratch the surface.
Again, there just aren't enough data points yet. I'm sure you could explore the right regions on Earth, using the right (wrong?) instruments, and conclude that life couldn't exist here.
But it is also quite possible that Mars is simply an environment which is completely hostile to any forms of life.
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