[WereBull]

Well, my Dad was talking about time travel, and leaping dimensions, so I thought about this. Maybe aliens are time traveling humans. Maybe their strange appearance is the effect of some kind of nuclear radiation, like there was some nuclear war in the future, and they've been coming back in time to prevent it. Sounds stupid I guess, but it's my theory.

[Strife]

Hmmm Well if they are as smart as most people "think" than they should know that if they change the pasted the future will change as well and they know there is a chance it will be much worse than there future, if you change the pasted how do you know there will even be a future? I mean everything is possible but if they are trying to prevent it, Im wondering why they wouldnt have made contact with us and told us to stop making what it is that will cause  a "nueclear war".

But the time traveling humans? now that could be possible i suppose i mean they abduct us and experiment trying to figure out what we are (sarcasm), we are as much different from them as they are from us, to them we are the aliens.

[Muerte]

  True.  If someone goes back in time and changes anything an interesting thing happens, this thing is refered to as a time paradox.

  http://homepage.mac.com/billtomlinson/newtt.html

  Read from the above for a little more perspective on the subject.  Though this is not the only source on time paradox it is a good jump off point.  To elaborate upon your theory however you could theorize that instead of time traveling they are instead inter-dimensional travelers.  In their dimension they have developed in a differing physical and mental make-up.  Purpose of abduction?  To understand the physical make-up of our world in an attempt to find a new home world.  After all, any sentient creatures desire is the multiply and expand its' influence.

[Thundergod]

I could see it as a vague possibility. I find the possibility for actual E.T.'s visiting a lot higher then Mutated time traveling humans.