Distributed trade escrowing seems like something that could be addressed using the existing Bitcoin Script mechanisms:
https://en.bitcoin.it/wiki/Scriptin conjunction with public API's from payment services that do not allow chargebacks (e.g. Dwolla, as I understand it).
When the network has matched up a buyer and seller for a trade, the seller's client initiates a BTC transfer using a script that only allows them to be unlocked upon receipt of verification of a USD transfer from the buyer's Dwolla account to the seller's Dwolla account (the transfer would include a special "comment" containing a hash code identifying it as corresponding to the current transaction). The "timeout" mechanisms could be used to send the BTC back to the seller if the Dwolla transaction was not verified within some time window.