In fairness to the Drake EQUATION, those who understand it do not use it as PROOF of life on other planets. It is, instead a TOOL to show that, given a very large number of stars, even if the individual circumstances that allow life to exist on EARTH are improbable, you are LIKELY to have a reasonable number of OTHER "HITS".
(odd capitalization in tribute to original post)
Here is a link to the equation, courtesy of Nova:
http://www.pbs.org/wgbh/nova/origins/drake.html
Will, you allude to a question that I have always pondered - why do we assume that certain conditions must exist for life (e.g., water)? Is it not possible for life to exist without it?
Actually, a quick google showed a thread in another board on this subject, which you may find interesting:
http://www.bautforum.com/showthread.php?t=7865
Contrary to Will, however, I would expect the possibility that life could arise in other forms to significantly increase the likelihood of there being life on other planets.