Help!

I have a large complex workbook with several pages, involving lots of
complex formulas and lookups, etc.

For some reason, on one of the pages, the formulas are showing up in
particular cells instead of the results. There are no indications of any
errors.

I'm currently using Office 20078 under vista, but I've had this problem with
this particular sheet in earlier version of excel. I've tried turning of
and on the display of formulas - that doesn't fix it. Re-formatting doesn't
help (either does the format painter). IT drives me crazy. I think I've
fixed it before by copying the formal to a new book and pasting it back,
although that doesn't appear to resolve the problem any more. Any
suggestions on what could be causing this strange behavior?