This Apple Security Update is for Mac OS X 10.6 Snow Leopard and 10.7 Lion. I believe at this point we can forget about further 10.5 Leopard updates. But emergencies do happen.
Here is where you can read all about it:
What's it all about? It's about the CRISIS of fraudulent SSL security certificates issued by the DigiNotar in the Netherlands, as well as other certificate authorities.
This will NOT be the last of the security certificate revocations. Expect another from Apple in sort order. The full extent of this CRISIS is still unknown and has most certainly not been stopped or contained. This may end up being the biggest Internet security crisis of the year.
Read more about it in my detailed article to follow.