What's the worst-case delay for BTC->USD? For big exchanges like mtgox, what is the longest delay time seen between ordering a sell and closing it?

In other words if I do an emergency sell of a large amount of coins because the exchange rate is dropping by the second, how much value might I lose while waiting for the sell order to complete?

Are there any exchanges that basically guarantee that the price in USD of my sell order when I place it will be the amount in USD I receive no matter how long their system takes to complete it?
When the BTC funds are on your MtGox account, a market sell order is nearly instantaneous and so done in a few seconds, dependent on the amount you buy.

Depositing Bitcoins to MtGox requires 6 confirmations before they show up in your account. 6 confirmations means 6 blocks to be mined since your transaction, this will averagely take 1 hour to complete.

The time for withdrawing your US dollars will strongly depend on the withdrawal method you want to use.

Note that there are many other exchanges and so this answer only holds for MtGox. It's likely that other exchanges require less confirmations and/or have faster withdrawal methods.