In the following analogy I have failed to take into account the Relativistic Doppler Effect which would cause "me" to age more slowly than "you". Since the speed of light is said to be independent of the motion of the observer the Doppler Effect, which may imply the opposite, is hard to accept.
The problem I have may lie in my idea of what "independent of the motion of the observer" means. But I digress...
Disclaimer: Someone else may want to
expand on this in a more formal,
correct, and scientific answer; but
here is my layman's idea of
theoretical time travel (well,
technically what i'm describing is
time dilation). It is probably wrong
but this "understanding" appeases my
Light is tricky. And relative. It "travels" at a constant speed (180000 mi/s) relative to the OBSERVER.
Here is my home-brewed analogy on the strangeness of light.
Lets just pretend that light moves at 10 miles per hour (or km/h if that's your thing) for simplicity's sake (I have a hard time comprehending just how fast 186000 mi/s really is).
If I'm driving my car toward you, with my lights on, at the speed of light1 (10 mph) how fast would the the light from my headlights be moving toward you? 20 mph, right? That would make sense...but it is not the case. The light from my headlights would travel at the constant 10 mph - relative to your perspective.
Now, relative to my speed (10 mph), at what speed would I perceive the light moving away from my car? 10 mph.
Hey, its light... it can do whatever it wants!
Now, I'm going to drive right on past you at my speedy 10 mph. And lets say instead of tail lights I've got a big digital clock shining back at you (its a 24-hour clock, where 00:00 = 12:00AM). Tick Tock. You never want to be late and don't want to rely on my clock so you have your own just to make sure mine is correct at all times.
Okay, think about that.
Tick Tock... I'm now 10 miles away from you still travelling at the speed of light. The hour hand on my clock changes from 00:00 to 01:00. The light zooms toward you at 10 mph. Meanwhile, I'm still driving away from you. Another hour passes for me and my clock changes again from 01:00 to 02:00; I am now 20 miles away. At that exact moment you perceive that first change from 00 to 01. Now you think to yourself, "Man, his clock must be slow!!! Its an hour off according to my clock!" Your clock says it is 02:00 (and so does mine, but you haven't perceived that yet).
So you wait another hour and look back up at my clock expecting a change. But nothing happens. You wait another hour and BAM, my clock finally changes from 01:00 to 02:00. But it is now 04:00 on your clock (it's 04:00 on mine too, but again... you haven't perceived that yet).
I suddenly and instantaneously turn around (impossible, yes; but I'm traveling at the speed of light so I'll do what I want). I'm coming towards you at 10 mph. My light (which beaming back at you telling you it is 04:00), from your perspective, is also traveling at you at 10mph. Since I am 40 miles away it should take me, and my "04:00" light, 4 hours to get back to you. So, 4 hours later, when you see my "04:00" light, I'll show up.
So, I appeared to either have a slow clock, or be 4 hours in the past (or 4 hours younger depending on how you look at it).
Here's the catch. If I had been monitoring your clock I would have witnessed the same time dilation. Your clock would have appeared to be slow and you would have appeared to be moving through time more slowly than me. (That's the "relativity" thing kicking in)
Here is brain thumper: what if I could go faster than the speed of light2?
So, it is not really time travel... but it is about as close as we are going to get until we figure out those darned wormholes3. :-p
 Einstein pretty much said we can't really travel at the speed of light under our current understanding of physics. (This kid may have something to say on the matter, though.)
 AKA "FTL" according to Stargate
 AKA "Hyperdrive".
So, about wormholes: If we could travel to the other side of the universe and then look back on ourselves the light we would see would be from a long long time ago (in a galaxy far far away).
What if we got in our super-speedy FTL spaceship after having been instantly transported via wormhole to the other side of the universe? Could we show up back on earth before we left?