When creating a bitcoin transaction, you have to choose which coins to use in them. The standard client does this in a way to avoid unconfirmed inputs and minimize the number of inputs and amount of change involved. Since not all coins are created equal, I may have other preferences (like spend unconfirmed coins first to get rid of them, or not cross inputs between different trading partners). Is there any way I can have more control over the process?
Currently this is not possible with the standard client, other than making separate wallets. There is a patch for coin selection on github, which was very promising. So this feature may make it into a future version.
Old question, so answers are outdated. Anyone reading this now: bitcoin-qt has coin control features that let you choose whichever txin you want to use.
Check out Armory, a new client that
uses an algorithm for coin selection which can be optimized for anonymity or minimal transaction fees.