To reproduce the problem here are the steps:
1) First create a function in a module
2) Save that worksheet as an xla addin
3) Now register that worksheet as an excel addin
4) Now call that function in an array formula (use Cntl+Shift+Enter)
5) Goto Tools/Share Workbook...
6) Click on checkbox to allow multiple users to edit it
7) Notice the error that pops up

It says:
"Cannot share 'Book1' because it contains array formulas that are linked to other workbooks."

Now this error is pretty descriptive and it completely describes the problem correctly. But here are the 2 questions:
1) Why was this not a problem in Office 2000?

2) And if this is really bad then why can you do the following:
a) Goto Tools/Protection/Protect and Share Workbook...
b) Check Sharing with track changes
c) Hit OK
d) Goto Tools/Protection/Unprotect Shared Workbook...
e) Notice that Tools/Share Workbook... checkbox is checked which you were not able to do before