I too am having issues with my workday formulas.  I keep my holidays from the
previous, current, and next years on a separate file.  My formulas link to
the cells containing the values for the holidays for all 3 years.  This
eliminates the need to update all of the cells in all of my spreadsheets
containing workday formulas.
After upgrading to Excel 2007, I have been having problems.  Thinking is was
my lookup formulas, I started copying the various source spreadsheets to the
main spreadsheets as subsequent tabs.  This has seemed to work on most of my
files, but it's caused problems of a different sort:  how to replace the
values of some frequency w/out losing the reference to my links.
Unfortunately, my workday formulas are still causing problems.  The problem
happens randomly w/out any rhyme or reason.
I cannot find a special add-in that would fix this, so I'm wondering if it's
a conversion issue from 97-2003 files to 2007.
I need answers to this problem SOON!  Re-work is taking up too much of my
time.
HINT:  One thing I did start doing is finding a cell that is outside of my
spreadsheet and pasting the workday formula I need into it, but putting an
apostrophe in front of the formula.  This now looks like text, so Excel won't
replace with "=#N/A" or "=N/A".  Anytime I need to fix my spreadsheet again,
I simply copy the formula (minus the apostrophe) into the appropriate cells.  
This is far from a perfect solution, but it saves me a lot of time.