Think Tank
This must be one of the most melodramatic posts I've ever read. "Can a caster use a Greatsword and still cast?" Why not? There are no rules that suggest otherwise. In fact, I think that the rules all but beg us to be so creative....
The SRD has this to say about somatic components and two-handed weapons, respectively:
Somatic (S)
A somatic component is a measured and precise movement of the hand. You must have at least one hand free to provide a somatic component.
Light, One-Handed, and Two-Handed Melee Weapons
This designation is a measure of how much effort it takes to wield a weapon in combat. It indicates whether a melee weapon, when wielded by a character of the weapon’s size category, is considered a light weapon, a one-handed weapon, or a two-handed weapon.
....
Two-Handed
Two hands are required to use a two-handed melee weapon effectively. Apply 1½ times the character’s Strength bonus to damage rolls for melee attacks with such a weapon.
Now from what I understand here, the two-handed rule only applies to melee, and has no dominion over how a character is holding the weapon, or with what number of appendages.
So, with that in mind, consider the time that it would take to remove, and replace your hand on a weapon held in both hands, if the weapon's size and balance were inconsequential. You might have come to the same conclusion that I have, being that the aforementioned time is utterly negligable, as these actions can be performed within a fraction of a second.
So one could even suggest that a character could, Gary Gygax forbid, hold the weapon off-handed for a few mere moments for whatever reason during his/her turn, without having to announce a series of actions (free or otherwise) to "ready" the weapon again, the time to do so being so negligable. And therefore the amount of time the character's hand could be removed (again, for whatever reason) would only be limited by the time within a round, and the actions that the character could perform (one-handed) within that time.
Now, looking back at casting in general, a spell with a casting time of 1 standard action normally takes up an amount of time short enough that it can be coupled with as much as a 30 ft. movement (in the case of most medium creatures). This includes the time that it would take to retrieve material components, recite verbal components, and perform somatic components. It would stand to reason then that these actions can be performed quickly and with general ease (conditions permitting) and that combined they require no more than a clear voice and a single free hand.
With these assertions in mind, it follows that one could, in fact, cast a spell while using a two-handed weapon (i.e. a greatsword) without sacrificing their ability to defend themselves or threaten others.
However, it's your house and your rules. To compromise, I would be willing to assert that a caster could use a full round action to cast while wielding a two-handed weapon. That meaning that it is too taxing to hold a weapon of that size one-handed while casting, and that a caster could rest the end of his staff/greatsword/etc on the ground with a partial action (not losing control of it entirely) and spending a second partial action to ready the weapon once casting is done.
Mind you, that still seems harsh, IMHO.