Proposed Solution
Either have a new operator for 'safe' division (someone told me TM1 has this), for example '\'. So, where n=1 and (d=0 or d=null), the expression 'n\d' returns null
..or, just return null instead of infinity when a division by zero takes place. I have never, ever been in a situation where returning infinity has been useful, it always needs to be handled. So why not have SSAS handle it automatically for us? OK, so it's a bit of a departure from the normal rules of maths, but already in MDX the expression
0=null
returns true, which is a good thing, so why not have
1/0
return null instead of infinity? Presumably this might also allow you to do some more optimisations inside the formula engine too.