wrote...
....
But it was my understanding that CELL("filename") was not volatile.
(Sources: http://www.decisionmodels.com/calcsecretsi.htm and
http://support.microsoft.com/kb/211370/EN-US/)
....

Don't believe everything you read. Testing is always best to confirm.

In this case, create a nonvolatile udf like

Function nada()
Static n As Long
n = n + 1
MsgBox n
End Function

then enter 2 separate formulas,

=CELL("Filename")&nada()

and

=nada()

Now clear the cell containing the latter, and the former recalcs. Undo
the clear. Now clear the cell containing the former, but the latter
doesn't recalc. Sure looks like CELL("Filename") is volatile despite
what Charles Williams's site may say.