One more noteworthy bit of information about Automatic Clearing Houses. Checkout Authorize's eCheck program at
http://www.authorizenet.com/solutions/echeck.php. One would be able to build upon the code of the authorize-gateway package which processes credit cards through Authorize.net.
Both packages would have the same type of connection to Authorize.net as well as the API. The difference is in the information exchanged between the gateway and Authorize.net.
/Bart