Am I missing something?
-Wayne
No, it looks like you pretty much got it. The client assumes that any coins sent are from an untrusted sender, so does not consider them mature for 200 blocks or so; basicly the next day. But the receiver should be able to consider them good on his or her own faith of the sender.
Actually the current client considers regular transactions as "unconfirmed" for just 6 blocks (nominally 1 hour). You can still spend them before they are confirmed and if there is a problem then your dependent transactions will remain unconfirmed too.
The special transactions for newly generated 50 coin creation transactions are held back for 120 blocks (20 hours). You cannot spend these till they are released.
Thanks for the correction.