Personally, I think a pilot should be able to put the plane down on grass and the only way to learn is to experience it (heck, why don't we learn how to fly planes altogether in simulators?????
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Saying that, a Boeing 737 landed on grass in British Columbia:
"On September 21st, 1972, Hope Regional Airpark was host to an amazing event! A Boeing 737 landed and took off several times on our 4,600-foot grass runway.
Hope Airpark was chosen by the Boeing Corporation as the perfect laboratory to test the 737's performance on a grass field.
The plane was fitted with special low-pressure tires, and the runway was wetted by tanker trucks to test performance on wet grass.
Resulting tire ruts left by the 90,000-pound aircraft were measured and recorded by Boeing engineers.
The Boeing pilot was Lew Wallick Jr."
http://www.hopeflightfest.com/photos.html - scroll to the bottom of the page to see photos.