Where to find the Mac OS X Lion security configuration guide?
I have it on good authority that the Apple Security Guides take a long time to compile and produce, and publication trails the release of the operating system by some time. The same people working on that guide are also producing documentation for various security certifications and other purposes. Once the guide is released, it will be available at Mac OS X Security Configuration Guides

With Apple's recent decision to move to annual OS updates, I wonder if we'll ever see a current security guide again.

Though overkill for most computer users, the US National Security Agency also publishes hardening guides for Mac OS X and other operating systems. (They haven't published for Lion either, and their own guidelines are very likely an expansion  of the procedures Apple recommends.)

When considering the NSA advice remember that security is always a balance between safety and usability and that most of us aren't protecting secrets of State. If you don't know why you're doing it, and have a good reason to believe you need to do it, don't!  There are relatively few users who need to lock down this tightly, and the negative effect on usability, and in some cases functionality, can outweigh the benefits.