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.
I would beg to differ on this one. Despite the fact that Zimbabwe dollars are running crazy inflation wise, then people could still trade them for the most basic needs in their life, food. (well maybe not a fixed amount of food, or good food, but still food).
If we cant convert BTC to food, BTC is just an alternative currency without any price comparison and will truly only be remembered as the "geeks" money. We need price comparison because we are decentralized and we need price comparison because I dont see my local diner or pizza place accepting BTC anytime soon.