You can use Count function:
=count(value1, value2, ...)
Make sure you place this formula in a cell outside your filtered rows/columns otherwise it will be hidden when you do a filter.
Also:
In 2007, right-click on the status bar (at the bottom) & select 'Count' from customize status bar. It will give the count of number of cells you selected (even for filtered cells). This would be just for your reference & wont show on the spreadsheet.