Yes, it was a practice bubble that collapsed. It only caused one serious injury, a broken back, which may be very serious. Ironically, the whole collapse is on film. Some of the players were practicing, so there were people with cameras there filming them. Here's a link to the video.
http://news.google.com/news?hl=en&q=...title&resnum=1
Pretty scary!
P.S. Well you have to click on the link on the page = Video: Raw Video: Inside the Tx. Dome Collapse. Sorry, I don't know how to post the video here.