I just found out that my new Airport Express can work as a time server (NTP server). Looks like a hidden feature, because I didn't find any mention of this functionality in its documentatiion or on Apple's support site. So I set it up to synchronize its clock with time.apple.com. After that was done, I set all my local machines to use this Airport Express as their time server.
Nice little feature and you don't need to drill holes in your firewall for UDP port 123 anymore.
[robg adds: The setting for synching the clock on the AE (and regular Base Stations) is somewhat hiddne. Go into the Airport Admin Utility and click on the Airport tab, then click the Base Station Options button. You can also change how the activity light behaves in this section -- always on, or flashes on activity. I haven't tried using my AE as a time server yet.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040809162009908