So, last night I took an offline wallet, stuck it in the Bitcoin data directory, fired up Bitcoin.exe, sent some coins, and immediately shut down the client. This morning I realize that the coins never actually went anywhere (confirmed by both the client and Block Explorer).
My question is this: when you send coins using the standard client, how long do you have to wait before it's safe to shut down the client? Until it shows up in Block Explorer? Until the client shows the transaction as confirmed? Both?
It depends how sure you need to be. What probably happened in your case is that the client never actually sent the transaction anywhere or the transaction was forced to be so extremely low priority it never got anywhere (you may have handed it only to peers who refused to pass it on). As soon as a normal priority transaction hits the network, assuming it is not part of an intentionally attempted double-spend, it should be confirmed eventually even if the client does nothing.