Look at the way how Bitmarket works:
The (Bitcoin) seller sends his Bitcoins to the Bitmarket escrow service
The buyer directly wires the Bitcoins to the seller
The seller releases the coins in the escrow
So you don't have to deal with any real money at all.
Another nice thing is that you can operate such an exchange 100% anonymous over Tor - you just need to somehow gain the trust of the community.
+1
Trust can also be facilitated with online reputation rating schemes like those used at @bitcoin-otc or ebay. Also, to echo your comment, one doesn't need to convert BTC to national currencies when he can trade it directly for the good or service he is seeking.