You can now buy and sell BitUSD and BitCNY for Bitcoin on
http://blocktrades.us (and also in the BitShares webwallet via the built-in bridge under the BlockTrades tab). We also theoretically support the same for BitEUR, but there's currently no markets to provide us with pricing discovery for it currently, so we'll have to wait for that market to pick up.
In the coming weeks we plan to introduce a few interesting new pairs (for some new coin types), but in the meantime, if you're looking for a particular direct pairing among our existing coins that you think would generate regular traffic, please us know.