I found this in the TSA website:
TSA is also modifying the rules associated with carrying breast milk through security checkpoints. Mothers flying with, and now without, their child will be permitted to bring breast milk in quantities greater than three ounces as long as it is declared for inspection at the security checkpoint.
Breast milk is in the same category as liquid medications.
When carrying formula, breast milk, or juice through the checkpoint, they will be inspected, however, you or your infant or toddler will not be asked to test or taste breast milk, formula, or juice. Our Security Officers may test liquid exemptions (exempt items more than 3 ounces) for explosives.
When traveling with your infant or toddler, in the absence of suspicious activity or items, greater than 3 ounces of baby formula, breast milk, or juice are permitted through the security checkpoint in reasonable quantities for the...
at 9:57 PM on Oct. 27, 2009