i doubt that'd even compile.. it'll probably say something like 'assigning void value should be ignored.' void isn't nil, it's just nothing.
not sure what you're trying to do, but you can differentiate between multiple buttons calling the same action with the tag instance variable - give each button a unique tag value, then do something like
switch ( [sender tag] )
//blah blah blah
//blah blah etc.
You could obviously call your own methods and return whatever you want from there.
So I finally got back to my OS X box, and yes, you can set a variable equal to the return value of a method that returns void (or at least, IBAction). Essentially, you're setting the variable to nil. That's not a problem.