Ice Cream Sandwich has arrived on the GALAXY S II with Telstra, and there's only a few steps between your old Gingerbread life on the GSII and your new Ice Cream Sandwich-rocking life. Do it now!
You'll need to visit this website if you want to do it, as there's a few steps you'll have to do. If you're still running the older Android 2.3.3, you'll need to upgrade to 2.3.6 before you can upgrade to ICS 4.0.3. Other than that, have at it!
It can be done OTA (over-the-air), or you can chose to upgrade it through Kies. This method would require that you backup all personal data, just in case. Before any update, I would suggest backing up personal data, it's always better to be safe than sorry.