I'm assuming you are talking about when someone looses their wallet because hard disk failure or some other reason.
Someone else might correct me, but I believe they remain in existence, but no one has the private key to spend them. So essentially they still exist, but are unspendable.

Yes but it has helped me get some posts, most of the newbie threads do not contain things I'm interesting in commenting on.

I signed up so that I could post some feedback about btcquick.

Other than that, I first knew about bitcoins back when they where $5/BTC and people where wondering if they where going to fall too low and would be abandoned. (especially by SR)
It is a shame I never bought many back then (or mined when ASIC mining wasn't even in the picture), I would have made a nice profit. Even if you do not whitelist me early I'm only ~2:30 away from being free, but it never hurts to ask. lol