Warning! Pull DigiNotar Root Certs!
In September 2011 Apple pulled the DigiNotar Root Certificates on Mac's with a Security Update for 10.6 and 10.7 machines, they haven't issued a update for 10.5 machines.
However Apple did release updates for iTunes and QuickTime for 10.5, 10.6 and 10.7. đ
What this means is if you visit a secure site or download software thinking it's from a secure site, it might be using one of the compromised certificates and compromise your machine or your vital information.
You'll have to pull these compromised certificates yourself manually in Keychain Access.
How to go about this precisely I can't tell you as I'm on Snow Leopard 10.6 and don't have a need to pull any root level certificates, sure someone can tell you how to go about the process.
Another method is fresh install & upgrade to 10.6, if you have a Intel based Mac that is.
Snow Leopard is much faster than Leopard it seems (using fresh install & upgrade), strips out PPC code and it gets video card speed improvements, I've tested the difference on just one machine though. It's a notable difference.
My advice is unless you have a 2011 machine or later, with 4GB of RAM, to leave Lion for a new machine. It will run on a Intel Core 2 Duo or later, just not as fast or perhaps as well as Snow Leopard will, and no Rosetta neither, like what Snow Leopard has.
You can read more of the DigiNotar compromise.
MacBook Pro, Mac OS X (10.6.8), 17" i7, XP, Vista, 7, Linux(s)