Re-visiting anti-trust legislation: "too big to fail"

Insurance giant AIG facing possible breakup

I am very definitely NOT an expert on anti-trust legislation!
What I learn from Wikipedia is this:

United States antitrust law is the body of laws that prohibits anti-competitive behavior (monopoly) and unfair business practices. Antitrust laws are designed to encourage competition in the marketplace. These competition laws make illegal certain practices deemed to hurt businesses or consumers or both, or generally to violate standards of ethical behavior.

The original intent of anti-trust legislation was to "encourage competition in the marketplace".

Given the underpinnings of the current mess - with so many financial institutions deemed "too big to fail" - perhaps it's time to revisit anti-trust laws with an eye more towards practices "deemed to hurt" consumers. Among these I would today count practices that encourage entities to become "too big to fail".

I note that Citi is divesting itself of large chunks of its business - making itself smaller.
Now we learn than AIG may be headed in the same direction.

Perhaps if they'd not become so big in the first place, we'd now be spared the $750Bn bailouts.

I'm guessing that the original intent of the various mergers & acquisitions that created these super-entities was marketplace efficiency, and from a pure "efficiency" standpoint it's likely to be difficult to argue against such mergers and acquisitions.
BUT - when the resulting entities become such large players that their individual failures threaten our entire economic system, something is out of whack. (See, e.g., Lehmann Bros.)

Could rational legislation be crafted that would prohibit any corporate entity becoming "too big to fail" without seriously impairing market efficiency?

I don't know... but it seems worth considering.