The Apple USB modem works fine with my iMac under OS X Tiger and OS X Leopard.
The Apple USB modem doesn't work with Bootcamp/Windows XP even after updating the Windows drivers from the Leopard DVD.
This appears to be a problem with Bootcamp or the WinXP USB modem driver. Windows Drivers Update does not indicate that any more recent modem drivers are available.
Given that Leopard includes the WinXP USB modem driver, I'm uncertain how to interpret the following statement on Apple's website at:
http://docs.info.apple.com/article.html?artnum=303262
Does the Apple USB modem work on Windows machines?
The Apple USB Modem is not supported on computers running any version of the Microsoft Windows operating system.