PS. I think the clock would turn twice as fast on the way back...
My intuition tells me that it would be turning way faster than twice as fast. Think of it: as you approach the speed of light the clock appears to move progressively slower until it stands still when you reach the speed of light going away from the earth. Of course, this completely ignores the question of perspective and the fact that the clock would appear infinitesimally small at this point--but I digress. Standing completely still, the clock appears to be moving normally. As you accelerate and begin to approach the speed of light it speeds up. Long before you even get near the speed of light it appears to be moving many times faster than twice its normal speed, because you are "plowing through" the photons which bring you updates of its movement at such a rate that it appears to be moving very fast. By the time you approached the speed of light it would be a blur of movement and you would see thousands or millions of years flash by you within seconds.
Of course this whole discussion also ignores the fact that Einstein's theory of relativity determines that anything which approaches the speed of light becomes infinitely massive, which probably means that you basically would fill up or consume the whole universe (including the clock you were looking at), which is probably why only light goes the speed of light because any massive substance would do this.
I don't know a lot about the minutia of physics; this is just stuff I learned from reading about it.
Carl
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