Absolutely. If the universe is truly infinite (not just immense!), it is impossible for us to be alone. Although the conditions for life on a planet are very specific, there has to be somewhere other life. In fact, using simple logic, there must be an infinite number of planets with life on them, unless there is a point in the universe where there is nothing but darkness, which could be the case.
Also, it is impossible for two given universes (within a tetraspace) to be exactly parallel, and there must be portals somewhere in our universe (again, an infinite number of these portals because tetraspace is also infinite). So, aliens from other universes - with completely different laws - could be in our universe after coming through these portals. (one theory is that black holes are these portals, but I think that the portal would be a constantly moving, infinitely stretching plane.
Whether or not they have visited earth, I cannot say. The evidence of them visiting seems to point toward such an answer, but considering the supposed law that nothing can travel faster than the spped of light and the fact that earth is incredibly far away from any planet whose surroundings meet the conditions for sustaining life, it would seem impossible. Despite this, they probably have - somehow. Perhaps they have evolved to travel at incredible speeds and/or developed longer lifespans.
I would suggest you watch the movie, "Alien Plant".
In short, my answer is a resounding yes.
