Basically, there are two schools of thought among the astrobiology crowd: the humanoid school and the "whatevernoid" school.
Originally Posted by plant
The former argue for necessarily very closely convergent evolution -- that is, that any intelligent alien would very closely resemble a human: "one head, two arms, two legs... that could be anybody", or in other words, think Star Trek -style odd skintones and funny foreheads at the most exotic.
The latter recognizes that there are probably some "evolutionary imperatives" (such as ability to sense electromagnetic radiation AKA "sight", bodily symmetry in some form etc.) that an intelligent alien would most likely need to have, but beyond that evolution is just too undirected a process to produce more fairly hairless apes on other worlds. So something that at least outwardly appears rather like the Xenos in Alien series might be in the cards, as well as possibly even more exotic-looking sapients.
The problem is that tho both can support their view with science, we don't have any samples of alien life to work with yet so it's still very speculative work they are doing. So, after long and careful deliberation, one must come to the inescapable conclusion: "I dunno".
The dog, the dog, he's at it again!