I've thought of that, but I doubt most DMs would let Ghost Touch have that effect. The power just lets a weapon touch and be touched by both material and incorporeal creatures. Even if it can switch back and forth on that kind of time scale, how would it know to turn incorporeal when it's being Sundered, but not when the wielder is trying to parry a blade?
If you allow this, you open the door to having Ghost Touch weapons ignore all armor. After all, it would be "most advantageous" for the weapon to pass straight through the enemy's plate mail, and then turn solid while already inside his body. Not cool, especially for just a +1 equivalent enhancement.
IMC I assume that the weapon is both corporeal and incorporeal at the same time. That has the same effect of letting it strike corporeal and incorporeal creatures, without the complexity of having it switch back and forth.
FTR, note that the Ghost Touch enhancement is effective against incorporeal creatures, not ethereal ones. (Yes, there is a difference.) This means you also can't use a Ghost Touch sword to eliminate the miss chance caused by Blinking.