MAn the online thing at one time is hard to implement! I tried it on my site, jdconcepts.net, and it just wouldn't work. I gave up after a few hours of solid trying!
It would be very cool though, and as soon as I see it (if it's done), you can be sure an email will be on its way from my PC to theirs'.
Is it? Store a list of usernames and timestamps in a textfile. On every visit check to see which users fall of the list (more than five minutes ago or so) and rewrite the file.
By the way, customize tends to drop out a lot over here during the day (which is the middle of the night in the US). Server not responding and stuff.
lol, that's nothin.... Immediatly following the re-launch of customize.org, but still before the server crashed completely.... I got a load time of over 480 seconds.... 8 fucking minutes to load a page!