This is my current project.
Basically it will be a trading plateform where your GnuPG key is used to send orders.
For instance, when you want to see your balance, you just enter your GnuPG ID, and the webserver returns the info ASCII encrypted. This should also make automation very easy.
To make an order, you'll basically just have to sign a phrase such as:
SELL 1000 BTC @ 0.6 EUR/BTC
And post it in a textarea.
The webserver will NOT do any actual bitcoin or banking transfer. It would be account based.
Deposit and withdrawal of bitcoins will be free but will have to be done manually.
Idem for withdrawals of Euros (I won't do USD). You will NOT be able to deposit in euros, though.
To withdraw euros, you'll have to post your signed banking coordinates first. You may also post a signed postal address, so that you receive bank notes in mail. There might a small fee for this option.
I'll try to keep it as simple as possible, in the same style that my BTC/EUR exchange site.
I write it all in a bash CGI script. Because I don't like PHP and I suck at any other web programming language. Also, bash is universal, for almost everyone has one on his linux box.