I realize this has already been resolved, but for the benefit of those
who may need another solution, such as myself, go here for a method
that applies conditional formatting to cells that contain a formula:
http://www.j-walk.com/ss/excel/usertips/tip045.htm
The advantage is that it's truly automatic.
In my case I have a column of cells that either contain a value input
by one of several users or a formula to derive a value if none is
entered. I need Excel to apply different formatting depending on how
the values are achieved and keep this formatting updated as users
alter the cell contents. This way users viewing a printout will know
at a glance whether the values were entered or computed, without
relying on users to remember to format the cells manually as described
elsewhere in this message thread.