Best guess from previous experiences, I think Apple has stopped point releases for Snow Leopard. They usually do that when a new version of OS X comes out. Security updates should keep coming in until early/mid-2013. They'll release important security updates when needed. OS X.5 Leopard came out in 2007 and got an important security update in June 2011.
At $30, moving to Lion should be a no-brainer at this point. Most apps should be compatible by now. I usually wait until the second point release to upgrade.