Today's Wii system update confirms Nintendo of Europe/Australia getting ready to update the Wii Shop for WiiWare.
The message accompanying the system update in both regions state that "this update features support for the WiiWare download service planned for May 2008."
The US equivalent is slightly different: "An update to your Wii Shop Channel is now available. This update is needed in order to access WiiWare games launching later this spring."
Nintendo have yet to officially announce a date for the service launch, but with system updates already confirming May as a time to put on your downloading hats, it does seem likely for a release around then.
Thanks to C3 reader iFrit for the tip.