United States Security Improvement Act of 2006 - Amends the Defense Production Act of 1950 to state that the purpose of a mandatory investigation of a proposed corporate merger, acquisition, or takeover (transaction) undertaken under such Act is to determine whether the acquisition could affect U.S. national security. Directs the President, within five days after initiating a mandatory investigation, to notify each House of Congress of the transaction being investigated. Requires such notice to be promptly transmitted to specified congressional committees. Prohibits the President from taking any action regarding a transaction under investigation until 14 days after congressional notification. Requires the President to consider and take into account any comments on the transaction provided by any congressional committee.