Why does the Bitcoin client require at least one confirmation before I can spend?
There is no technical reason you can't spend it immediately. A transaction that is dependent on an unconfirmed transaction should be accepted by the network and should eventually make it into a block.

You will almost certainly have to include a transaction fee with the second transaction in order to get it accepted into a block immediately. Otherwise, you will be waiting some time before the second transaction gets committed into a block.