Adam Buenz, fellow SharePoint MVP, is by far the top dog in my book outside of Redmond on the subject of SharePoint security. Adam is the guy behind www.SharePointSecurity.com that is an absolute wealth of information. He's recently started a blog of his own: SharePoint Shelter.
One article he's got on his site (not his blog) is the unified provider framework, something that will make your life easier then creating a custom authentication provider (heard him say once that it only takes him 15 minutes to link it up with an Oracle LDAP store!).
I highly suggest if you're in the SharePoint blog feed collection business you subscribe to Adam's new blog. I've heard him spout off on various security subjects, admittedly a higher percentage are beyond me even back to my days with Secret clearance working for the US Defense Dept. (DISA) then I care to admit, but he's ALWAYS the guy I go to for anything security related with SharePoint.
So... here's my recommendation for your two-stop source on SharePoint Security: Adam Buenz.