There are organisms that can live in very human-toxic environments on earth, so that is possible. But, most of them are very simplistic monerans, so it gets iffy whether more complex organisms can form in them (esp. in conditions that are extremely harmful to life in general). They can survive partially because of their simplicity; there is not much to go wrong, so not much protection had to evolve.
There actually are some silicon based lifeforms on earth. They are monerans (bacteria), so nothing too complex.
But, I am not sure if silicon can perform as efficiently as carbon for more complex organisms. It might be possible though. Of course, that is the only other element that complex natural (ie: non-robotic) life could be based on, as it is the only other element capable of forming hugely complex molecules (100s to 1000s of atoms) as carbon can (though it does not do it as well). Such complex molecules are necessary for natural lifeforms.
[ July 18, 2003, 19:06: Message edited by: Imperator Fyron ]