Spells Centered on You

[FireLance]

a fireball is a poor example i think, as its not centered on the caster. my point is a medium creature has no center. If its corners are the center, then a caster should be able to change what conner the spell is on, as all corners are the center.

I wouldn't have a problem with that. Since a Medium creature is supposed to be moving around within his space (and not standing still in the exact centre of it), the corner which is considered to be the centre of the spell is the corner he spends most of the round closest to. I wouldn't have a problem with a spellcaster changing the effective centre of his spell every time he moved, or once per round as a free action if he does not move.