From the link
http://www.sheldonbrown.com/tires.html#width:
Most tires have a "maximum" pressure, or a recommended pressure range marked on the side of the tire. These pressure ratings are established by the tire manufacturers after consultation with the legal and marketing departments.
...
Newbies often take these arbitrary ratings as if they had some scientific basis. While you'll rarely get in trouble with this approach, you will usually not be getting the best possible performance with this rote approach.
Play around a bit and see what works for you. Personally, I weight about 135 so I've probably got overinflated tires
-murray