It seems that Apple's installer is just searching for "iTunes" in the process list and then making its install/no install decision based on those results. So if you're having trouble getting it to install, use Activity Monitor to look for "iLife-like-named" processes and kill them off, too.
[robg adds: I tested SizzlingKeys, and it adds only that one process to the list, so it certainly seems that this is another example of a semi-brain-dead Apple installer. They're probably just grepping on "iTunes" and running with the results.]

