A team of scientists seem to have proven it will be impossible to experimentally verify the possibility that at one point, far in the distant past, all the forces that make our universe tick were once one. I'm just about done with Warped Passages, by Lisa Randall, which discusses this exact situation. It seems that there is definitely a problem verifying any theory which relies on Planck-scale forces. However, if the universe contains more dimensions than the four we perceive, and they act in the way that modelers (string theory and otherwise) think they do, then it's quite possible we'll be able to concoct experiments to test unification that won't require obscene amounts of energy to work.

Hey, man, don't look at me. My head explodes every time I read a chapter of that thing. I sorta like it. :)