The next time Software Update asks you to update iTunes:
1. uncheck iTunes from the list of updates
2. click the help menu, search for "ignore update" and click on one of the options. (I don't remember exactly what it's called.)
It's a hidden way to tell Software Update to never bother you with iTunes updates again.
Warning: This will leave you exposed to any security issues in iTunes, so don't run iTunes after you've shut off its updates.