Have a friend that's running OSX 10.5.8 and claims it's still supported by Apple and has the same security patches as needed as any other version of OSX. Does Apple still support 10.5.8? What if any know security holes are there? Does Apple have a public statement on which OSX versions are supported, how, and what this means to the user, in the context of OSX 10.5.8?
You can see the complete list of Apple security patches at this page: http://support.apple.com/en-us/ht1222
You can see for yourself that the last security patch for OS X 10.5.8 was "Leopard Security Update 2012-003" that was released in May 2012.
There is likely to be a large number of unpatched security holes in Leopard at this point. However it is unlikely that Apple would publish such a list and I'm not aware of anyone else who maintains such a list.
Another issue is that third party software developers may not support Leopard anymore. For example, the current versions of Chrome and Firefox both require a minimum version of 10.6, while the latest version of Safari that runs on Leopard is v5 while the current version is v8. And the last time Safari 5 saw a patch on Leopard was also May 2012 according to the Apple support document above.