My son asked me this question the other day. So we looked it up, and found a few other facts as well._
The most recent salary increase, to $400,000/year from $200,000/year, took effect when George W. Bush became President. The President also receives a $50,000 non-taxable expense account._
The retirement benefits received by former Presidents include a pension, Secret Service protection, and reimbursements for staff, travel, mail, and office expenses. The Presidential pension is not a fixed amount, rather it matches the current salary of Cabinet members (or Executive Level I personnel), which is $191,300/year as of March, 2008_
P.S. I also found out that President George Washington was offered a salary of $25,000/year._
The above information was found at About.com.