MtGox is synonymous with 'bitcoin' at this point. Everyone who knows what a bitcoin is knows what that site is.
Are all those 20k accounts legitimate?
For about 20 minutes, they buy/sell buttons were non-responsive. You could click on them - but nothing happened. I have a suspicion (without evidence other than observation) that at one point today the buy/sell functions were disabled.
The buy/sell buttons always worked, as far as I know, but when the lag is an hour I can see how it would seem broken.
Failure Scenario #1 - Buy/Sell Buttons were not working. You would click them - but absolutely nothing was happening. On #MtGoxLive there were a number of us all experiencing exactly the same thing. A few believed it might have been a javascript issue.
Failure Scenario #2 - Once the buttons were activated/fixed - your order was placed in queue instantly, but remained "pending" - on #mtgoxlive, ;;goxlag showed lag time of about 600 seconds - My order went from pending to "executed" exactly according to ;;goxlag.