LOL @ HeavyDuty's Martian Stucco Face!!!
Matt-- true there is nothing saying the all species have to look the same. However, there are certain things that, based on laws of the universe (physics and such) are bound to be ubiquitous. Bi-lateral symmetry would be one such thing. Certainly life forms COULD have horizontal symmetry, but unless you're floating in open space, there would be extreme disadvantages. If a creature is walking on a planet, what would be the point of having feet on top of you as well as on bottom.
This is just a long-winded analogy as to why, in my opinion, it is LIKELY that alien species would have physical similarities to humans.
BTW... if life evolved just as easily in all environments (extreme heat/cold, methane atmoshperes, tremendous gravitation, etc), then why didn't it evolve on other planets in our system? Instead, it only evolved on a planet with very moderate climates and abundant oxygen and nitrogen.