I've found that whenever my machine goes to sleep, I lose my network connection. Unlike many other people, my machine automatically reconnects to the AirPort network, but the fact that I lose my connection is still a pain, especially if I'm connected to our company's Filemaker server. I found that MattMac posted the solution to this problem on the Apple Forums, which is simply to uncheck the "Sleep hard drive when possible" checkbox in the Energy Saver pane of the System Preferences panel.
I thought this might also be helpful for people whose machines don't reconnect to the network after sleep.
[robg adds: I haven't had any issues with losing servers I'm connected to when my PowerBook sleeps, but I also uncheck the hard drive sleep button on all my machines...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040214162448260