FDIC offers the following on their website: http://www.fdic.gov/regulations/resources/signage/
"On October 3, 2008, FDIC deposit insurance temporarily increased from $100,000 to $250,000 per depositor through December 31, 2009."
In the near future, FDIC will provide insured institutions with information regarding any changes to the official FDIC sign (teller station signs) and instructions for ordering new official FDIC signs (if changes are made). In the meantime, insured institutions may post the above statement, or affix a sticker with the above statement, next to the official FDIC sign.
It says "may post the above statement" so it appears optional, at least for now.