Because just about 30 minutes ago, that's what happened to mine. Now I must say that I've got an 'engineering sample' not a production unit, but when I went to turn it off, the button went down hard and stayed down - no popping back up and kind of a very, very soft rattle. I'm pretty sure it's broken, but I don't know as it's impossible to get the case open without scraping off several small rubber dots that cover four hidden screws. I'm not enough a hacker - and this is a VERY costly device (my 'engineering sample' cost me over $500, and that was without a 2GB card, softcover case or spare battery pack) - so I've now joined the ranks of those who are waiting for a replacement unit.
Who knows, maybe they'll send me a NAEB package deal with the softcover case and 1GB card to replace this one. Anyone know how long it takes???
I'm bummed out, 'cause now I have to go back to the very difficult-to-use Sony PRS-500!
Derek