Assuming that the transactionID is not modified by any peers, unconfirmed transactions (0 confirms) can be spent.  I have one this on multiple occasions.  If the transactionID is modified by a peer, then the transaction can be spent as soon as it has its first confirmation.  Of course, if that confirmation is from a block that gets orphaned, and the resulting transaction is then modified by a peer, then it would be safer to wait for 2 confirmations.  The more confirmations you wait for, the lower the risk that the transaction is in a block that will become orphaned.  Given this, at what point can you "spend them"?