I am not sure when this started happening, or if it happens to all machines, but it has become handy for me. Certain laptops, particularly older iBooks, when woken from sleep, can take a bit of time to restore their AirPort connections.
Generally, before it is up, I have attempted to load a webpage in Safari, and get the usual network error. In the past, I used to hit refresh repeatedly, until the connection finally came up and the page loaded.
You need not do any of this, however; just sit there, and as soon as the AirPort connection is up, Safari somehow knows to reload the page, all on its own.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20051201002900154