>> The function definition can use the standard Excel =IF(,,) function
>> to make a conditional result to stop the recursion.
>> The rest would be provided by the standard mechanism of function calls.
>
> I think Andy was talking about how Excel would support recursion, not
> how a user would implement a recusive function.  Currently Excel would
> throw a "Circular reference" error.

...unless you have checked the "Iteration" box under 
Options->Calculation. It's possible to then write a recursive formula, 
the classic example of course is factorial:

Cell A2: =IF(A1="",IF(A2<=1,A2,A2-1),A1+1)
Cell A3: =IF(A1="",A2*A3,1)

You can then type a number in cell A1, hit enter and then delete the 
number (the formula in A2 restarts the iteration when A1 goes empty). 
Cell A3 will then give you the answer.

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