One main quirk I've had with using Drupal (and other Open Source CMSs for that matter), is that when I need to do some heavy maintenance and take a site down, the default 'site offline' message looks pretty mickey mouse and though I'm proud of using Drupal, I'd rather have my own logo, if one at all, when people see that my site is down - I mean, it looks as if they've reached some page they shouldn't have and something's seriously messed up.
Well, you don't have to waste much time pondering these things like I have - there are two approaches in Drupal 5 that let you beautify the site offline message, and I recommend you read this post:
http://drupal.org/node/58562
About 2/3 of the way down the page you'll see mention of a simple template.tpl.php include which will redirect people to a custom/static page or URL but still allow you to access site login/admin pages. :)
Last note: supposedly Drupal 6 includes phptemplate theming for the maintenance page; I'll try it out and post back soon.