I have always been dubious about our form of life being the only kind there is and can be. While as the only type of life we actually know about, scientists have little choice but to base their search of extraterrestrial life on what we know of carbon-based life, it is rather premature to exclude other possibilities. So the possibility that life as we do not know it might exist right alongside us on Earth is rather intriguing.
It is almost certainly not the case that that there are multicellular life forms, or at least what would correspond to life forms, that are not based on familiar DNA, RNA, or even proteins. However, we have not microscopically examined even a significant portion of the entire Earth, and we may not even recognize totally different life forms even if they are staring us in the face, since almost all of our knowledge of microbiology is rightly focused on life as we know it. While it may be a quixotic quest, considering that any such alien life forms would likely be quickly gobbled up by more familiar life, it's a slim possibility. I would still consider a search for alien life on our own planet to be worthwhile: if discovered, it would really expand how we think about life, and it would also be evidence for life arising multiple times on Earth.
It could be interesting to hear what Paul Davies said about the topic in his lecture on February 15.