While I don't think human time travel is possible (again if it were we'd see buttloads of "time tourists")
I subscribe to the "Since time is a vector that naturally goes in one direction that's the only direction you can travel" theory.
Who says you can't jump forwards but not backwards? Could explain your lack of "Time tourists."
Heh, it's funny, that's kind of the reason I think that backwards time traveling might be possible. Since time probably is a vector, it must have a start point. While it might not be possible to travel backwards directly upon a vector, it is usually possible to get off said vector and intercet a previous point via bypass.
You know like the old "time is a river" metaphor. Sure the river of time might flow so shiftly and strongly that you couldn't possibly paddle up-stream, but why wouldn't you just paddle towards the bank, get off and walk upstream?
Again, that doesn't mean the HUMAN time travel is possible, just time travel in general. Just like technically speaking, you could probably travel to another dimension by passing through a black hole, but it will never happen because you would be ripped apart at the sub-atomic level as you approach the event horizion.