Syncing your iPad with iTunes will not cause an update/upgrade to iOS 5. You will always have to confirm that you want to upgrade either manually by clicking on the "Check for Update" button in iTunes or by confirming a pop-up if one appears.
Here's how to update:
http://www.ipadforums.net/ipad-faq/50385-how-do-i-upgrade-ios-5-a.html#post345719
Prior to upgrading to iOS 5 you might want to take a look at these the tips in the first post:
iOS 5 Upgrade Troubleshooting
FWIW the iOS 5 upgrade to my iDevices went without a hitch.
Hope that helps!
Thanks so very much