Well, by our current Earth-bound understanding of science, the typical answer is: "No, it is not possible, but it makes for great science fiction movies."
I asked this question, with silicon being the base for another possible life-form, because that seems to be the most popular suggestion out of all the elements we know about. Personally, I'm more curious about there being life based on other elements that we don't know about or haven't discovered - that is possibly out there, in ample amounts, somewhere in the universe.
Instead of me typing a bunch of subjective opinions that I generally enjoy doing, mainly on other websites and blogs of mine, I'll just provide a couple resources that are not supporting the theory of silicon-based life, but their reasons may provide even more questions and possibilities, depending on your imagination.
1) Conceivably, some strange life-forms might be built from silicone-like substances were it not for an apparently fatal flaw in silicon's biological credentials. This is its powerful affinity for oxygen. When carbon is oxidized during the respiratory process of a terrestrial organism, it becomes the gas carbon dioxide – a waste material that is easy for a creature to remove from its body. The oxidation of silicon, however, yields a solid because, immediately upon formation, silicon dioxide organizes itself into a lattice in which each silicon atom is surrounded by four oxygens. Disposing of such a substance would pose a major respiratory challenge.
Read more, here: http://www.daviddarling.info/encyclopedia/S/siliconlife.html
2) Silicon biochemistry: The most commonly proposed basis for an alternative biochemical system is the silicon atom, since silicon has many chemical properties similar to carbon and is in the same periodic table group, the carbon group. Like carbon, silicon can create molecules that are sufficiently large to carry biological information. However, silicon has several drawbacks as a carbon alternative. Silicon, unlike carbon, lacks the ability to form chemical bonds with diverse types of atoms, which permits the chemical versatility necessary for metabolism.
Read more about this and other exotic element-based biochemistries, here: http://en.wikipedia.org/wiki/Hypothetical_types_of_biochemistry
...Okay, now back to the original query, "Is Silicon-based life possible?" This concept has been applied to many fantasy and sci-fi flicks. For example, the original Star Trek series, episode 26, had a show that featured a silicon-based being. This particular show was called "Devil in the Dark." The Horta (a silicon-based life-form) is discovered by miners on Janus VI and every 50,000 years, all the Horta die except for one individual who survives to look after the eggs of the next generation. Anyway, the point is, on a planet with a totally different chemistry and available elements (many elements that could possibly be unknown to us), it is a plausible theory that life may not necessarily have to be limited to the carbon-based category.
I suppose that I could have changed the title of this post to "Does all life have to be carbon-based?" and it would have had the same effect. Hell, forget about the silicon-based life theories and the doubts therein. Just think about the possibility of all the other exotic elements that we may not even be aware of, that could exist deep within the cosmos. When it comes to the marvels of life itself, the mystery in what makes life possible, and the amazingly stupendous amounts of various types of life that we do know about on this beautiful planet, I'd say that anything could be possible elsewhere......
---End of Post "Is Silicon-based life possible?"