Formatting the cells in a pivot table worksheet is not a great idea because
refreshing the pivot table may or may not retain the formatting, especially
if it has to expand due to having more data.
It's better to right-click each field and format the "Field Settings"
"Number Format" to what you're looking for. Then that field will always
utilize that number format, even after a pivot table refresh.
Excel 2007 seemed to have no problem for me allowing 2 different currency
types in one pivot table.