« Yet more reason for replacements/nLockTime to be turned on, you would never need an « account » on mtgox that you had to deposit more than you were going to immediately use into.
In general long-term business relationships can agree to just « settle it in the blockchain later », but still do it in a way that neither can screw the other.
1) Do the « standard escrow setup »:
a) Write Tx A w/ output spendable by sig from you AND mtgox, don’t sign it yet.
b) Write Tx B spending Tx A back to you with nLockTime some time in the future and seq number 0.
c) Hand mtgox both these *unsigned* (don’t sign TxA until he has signed TxB), demand he sign them, get them back and sign them, broadcast them.
2) For quick withdraw, mtgox can do the same thing in reverse with you.
3) To start off, Tx B has seq 0 and gives all coins in TxA back to you. When you need to give mtgox money, you send him a signed replacement of TxB that is final, and sends some of TxA to him and the rest back to you. These are all offline, not sent to network.
4) As you need to give more money, you send new replacements for TxB giving more to mtgox and less to yourself. Again these are all offline.
5) Finally, sometime before the original TxB expires, mtgox should broadcast the last signed replacement you gave him (the one that gives him the most money).
In this scheme, money is tied up, but still cannot be spent by the other party without authorization from you. On the other hand, you can provide that authorization to them instantly offline of the blockchain. »
Par Hashcoin – 4 juillet 2011
Source : bitcointalk.org