...but no one has actually seen it?
EDIT: OK I got busy and googled it. From a SNOPES message board, 2004:
"My friend and his wife were in an accident where the airbags were deployed (a Jeep Grand Cherokee). She had her feet up on the dash. It didn't kill her, but gave her a nice "shiner". She is an older person and (as she says) her legs aren't able to bend enough to touch her head, but during that accident, they did. Her legs (and head) hurt for a long time, but again, it didn't kill her.
It is only one (but verifiable) data point".
And one from FIREHOUSE website forum, 2005 (car hit a deer at 35 mph):
"The male passenger was seat belted, but had his feet on the dash and the airbag deployed. Both femurs were broken midshaft. He resembled a christmas turkey. He was extricated and survived. The state trooper secured a ride to the hospital for the driver/mother and the rear passenger who was the grandmother, none of which reported any injuries. The grandmother had a massive heart attack in the emergency room two hours later and died.
The drivers airbag also deployed, but did not cause injury to the driver".
Might be hard to predict the trajectory of knees when bags blow, but the dearth of experiences tells me it can't be a pragmatically common event.
EDIT:
Xray of foot fx blamed on airbag:
http://www.flickr.com/photos/roentgenator/1384930412/
Here's an urban legend:
2001