Rob's article over on The Gun Zone does an excellant job covering both Title 18 section 930 and CFR 39 re:firearms in federal facilities. The only federal facility it's an absolute no-no that I know of is a court house. While the Post Office would like you to believe it's a crime, it isn't. Read the law, read the CRF, especially the part which says, "... Nothing contained in these rules and regulations shall be construed to abrogate any other Federal laws or regulations of any State and local laws and regulations applicable to any area in which the property is situated."
As to national banks, mine is one and they welcome legal CCW. Carry at the PO, BTDT - just this morning, in fact.