Hehe, my knowledge of biology is at best mediocre, so I posed the same question a little while back and I think Hazzard put it very well:
So as I understand it life could indeed be non-carbon based, but carbon is the most likely both due to the abundance and the chemical characteristics.
Whoooo! glad I wasn't the only one in that direction--
Considering that they may not be rocks, then Wonder if the ufo'ers would be disappointed if aliens turned out looking something like a jellyfish?