In as much as Mac OS X 10.5 ("Leopard") is having a semi-rough time getting out the gate, there are some workarounds that can help.

The guys over at MacFixIt have discovered that the use on non-ASCII characters, especially with third-party routers, can result in said networks not showing up in the Mac OS X AirPort menu.

Readers have offered the following steps to help rename the router from your Mac:
-Select the router from the AirPort menubar item or within Internet Connect (located in /Applications). The router is usually named after the manufacturer, e.g. "Belkin," "Linksys," or something like "Untitled."

-Open a browser and enter the address 192.168.1.1. If one browser doesn't work, try another. Some routers, for instance, cannot be properly configured through Safari, but can through Firefox.

If you've seen a similar error or found a useful workaround, let us know over in the comments or forums.