Jeff,
Describe exactly what you want to do. The short answer is that you don't put a formula in a pivot table, but you can calculate based on fields and items within fields and have the result appear as part of your pivot table. You can check out John Puckett's post and Mark W.'s reply below to see how this works.
So, what do you have in mind?
Jeff, there are 2 kinds of formulas that you can
use in a PivotTable -- Calculated Fields and
Calculated Items. For a descripition of each
see the Excel Help Index topic for...
1. Syntax for calculated field and item formulas
in PivotTables
2. Create a calculated field in a PivotTable
3. Create a calculated item in a PivotTable