After doing as much research as I could (very limited with the weak web
resources on this topic) it seems that xlfCaller is a Level 2 Macro call
and can only be called from a Function registered with a # at the end of
the type_text. Pulling someone else's code, I created a small function
that gets the cell formula and passes it back in an XLOPER. I
registered that function with the # and then I use xlUDF to call it and
get the XLOPER it returns in my main code. For the time being,
everything seems to be working OK.
Thanks for your efforts.

Rob Bovey wrote:
Hi Peter,

The problem is that Matthew is trying to get the function string from
within the function itself. As far as I can tell, xlfGetFormula does not
work when called from within a function and passed the address of the cell
(as returned by xlfCaller) from which that function was called.