My own two cents on this is that I personally don't think it'll happen.
It took a few years to get to 1GHz. Then a much shorter time to 2GHz, then 3GHz... and then what? Where's 4GHz? Instead we're getting mulitple cores, and 64-bit wordlengths instead of 32-bit.
To get processors measured in THz, you'd have to go up to 10GHz, then 50GHz, then 100, 200, 500, etc. all the way to 1000GHz: 1THz. I've read that even with 3 or even 4GHz, Intel and AMD were already having to deal with electrons not being able to move fast enough from one place on the die to another, because they were limited by the speed of light.

So I seriously doubt we'll see processors measured in Terahertz.
Tom