How do I suppress showing cents on a contribution page?
In Settings - Localization, try changing Monetary Value Display from:

%!i


to:

%!.0i


See: http://forum.civicrm.org/index.php/topic,20941.msg87684.html#msg87684

Please see the comments below by Andrew Hunt and Lobo - the above will get rid of cents/pence across the board, which may not be what you want. See Lobo's answer for alternative approaches. Trust Lobo. :-)