Download the Ultimate Wallpaper app http://www.precentral.net/app-gallery/beta/ultimate-wallpaper
Get the URL of your Google calender as described in this link:
http://www.google.com/support/calendar/bin/answer.py?hl=en&answer=37648
It's a long URL, so you could send it to you email on the webOS device and copy it from there.
Open the Ultimate Wallpaper on the webOS device and set it up with News and paste the copied URL into the RSS feed in the app.
Now you got your calender in the wallpaper
