I have a possible solution to Mac users.. If someone can figure out a PC equivalent and post it here I'm sure people would appreciate it!
To contain the .lrcat catalog file, create a password protected disk image (.dmg, mac only file) in Disk Utility.
Before you open Lightroom, open your .dmg and select your .lrcat file to load. Lightroom 3 also remembers this file path so you can use the 'recently opened' features, but the catalog will not appear unless the disk image is mounted.
To speed up the simple process even more, use a hotkey to launch your .dmg file.
I'm thinking about a more general solution: persistent smart collections to serve as global pre-filters, with option for password protection. This would allow one to implement discretionary viewing of all sorts, and do other things... If you really didn't want any photos to be viewable without permission, then just define a password protected smart collection that is all-exclusive, and lock it in...
See for more info and to vote: