Simply add this participant to any wave for which you want email change notices:

wave-email-notifications@appspot.com

After you add this the first time, you'll find a new wave in your inbox entitled "Notify global preferences". Visit that wave to set your notification preferences.

The notification messages are timely but contain few details—just an indication of who modified the wave and a link to the wave, so you’ll still need to visit the wave to see what has changed.

Within each wave, the robot provides three notification options:

• Email me just once (until I visit this wave)
• Email me on every update
• Emails disabled

Overall the robot works reasonably well, but I’d still like to see an email notification for new waves, not just changes to existing waves—at least until Google Wave becomes so ubiquitous that we need wave notifications for new emails. ;-)

Updates - 05 Jan 2010:

To clarify: Once the robot has been added, notifications are sent to all wave participants who have configured their "Notify global preferences" wave (not only to the participant who added the robot).

More importantly, there is now an even better solution that provides e-mail notification for all waves, including new waves, without requiring the use of a robot.