[benpbenp]

You may be surprised to learn there was debate on this topic from at least the high middle ages. A quick Google search found this write up, which seems to be a pretty good summary.

http://vox-nova.com/2009/10/10/on-multiple-worlds-and-multip...

EDIT: This is quite good too:

http://www.unav.es/cryf/extraterrestriallife.html