That's a very interesting direction. Other than the annoyance of requiring that someone have something in order to give something, it does seem to fit all the criteria for a social solution to fraud.
Well if you already have significant reputation & trust it is unlikely you will need to put up a counterparty deposit. Obviously is someone asks you for more of a deposit you are comfortable with you can not trade. If it becomes commonplace it will be just another negotiating tactic so that both parties feel their risk is hedged.
Interesting thing is that you can use blockchain to facilitate order updates too.
1) Mathew wants to buy a rig from me from me. We agree on a price of 150BTC.
2) Mathew wants to use escrow (smart idea) but I have little rep so he demands a counterparty deposit of 30 BTC.
3) I negotiate it down to 10 BTC which he accepts.
4) We create the transaction with 160BTC which requires 2 signatures to release.
5) Once I ship the product I sign the first half of a "paid" transaction (all 160 BTC goes to me) I include in the transaction the tracking #.
Now depending on how sophisticated Mathews's client is (and this doesn't exist yet) the block chain could notify him not just of the half signed transaction but decode the text message inside showing him "Hey mathew I shipped the rig. Here is tracking #. Be sure to sign your half once you get it".
The half signed transaction sits on his wallet until he either signs it or rejects it (the blockchain has no concept of reject, reject would simply hide that transaction).
Now this is one example but when people like me says Bitcoin NEEDS mult-sig to expand beyond uber-nerds this is why. multi-sig allows users to replace implicit trust with stronger methods.
Issue: wallet can be stolen
Solution: multi-factor wallets are much harder to compromise
How: multi-sig allows 2 factor signing (wallet + secondary key on smartphone for example)
Issue: seller can be scammer (trading requires implicit trust)
How: multi-sig allows a transaction to be "half completed" and frozen until consensus between buyer & seller is acheived.
Issue: online wallets can be scammers (mybitcoin)
Solution: variant on multi-factor wallet
How: multi-sig allows wallet provider to retain 1 of the two private keys and the user passphrase is in realtime converted into the 2nd private key
Issue: double spends can be dangerous but merchants want realtime processing
Solution: multi-factor wallet provides some double spend protection
How: Since a spend requires wallet provider key (not just user key) a double spend would require both wallet & user. If merchant doesn't trust user but trusts wallet provider (because say they are multi-million dollar company w/ real assets) then double spend by user alone isn't possible.
And the list goes on and on.