Skepticblog has some nice speculation – starting with this video which claims that, whatever aliens look like, they certainly won’t look like us.
Richard Dawkins wades into the argument to make the case for convergent evolution. Convergent evolution is the phenomenon where animals evolve to fill similar niches despite very different ancestry. Common examples are sharks and dolphins or bats and birds. There are (or were) whole menageries of marsupials (mice-like, mole-like, bear-like, wolf-like) that look remarkably like their distant placental relatives.
I would agree with him in betting against aliens being bipedal primates and I think the point is worth making, but I think he greatly overestimates the odds against. Simon Conway-Morris, whose authority is not to be dismissed, thinks it positively likely that aliens would be, in effect, bipedal primates.
Dawkins ends up concluding that
androids are fairly improbable, but not necessarily all that improbable
For my part, I’d have thought that octopus-like aliens are far more likely than human-like ones.