I know there were requests for this a long time ago (I'm still stumped that FB hasn't added it yet), so I just wrote it for personal use (real simple

) but then AI saw it and asked me to extend it a bit for use with NSN-Nuke. I started it and now it is finished, I think.
See In Action Here
http://ns3566.ovh.net/~earthfra/forum/
You can enable, disable it through the nuke preferences, there you can also set an optional message to explain why you temporarely closed the site.
If you are logged in as admin you can still browse around the site without limitations, you can even go login as a user. If you aren't logged in, you can still go logging in at admin.php, cause I've set the mod up that the message only doesn't show up when you are at the admin.php page.
If someone is interested, let me know. If not it will never be released for phpnuke and only for nsn-nuke.
Mighty_Y
[edit] Please test this mod so it can be released as v1.0.0, I don't exspect much bugs as it is a very simple mod
http://users.skynet.be/fa091229/Site_Close_Down_Mod.txt