The API function in MB (ModernBill -
http://www.modernbill.com) is really nice... It doesnt 'automatically' do it at signup, but you login to MB and go to the 'API Queue' section and it creates their account at the click of a button. Its a nice 'all-in-one' solution for resellers AND dedicated owners. Ive used it for about 4 or 5 months now and I like it a lot, check it out. Also, I found an AWESOME new support ticketing system that has live chat integrated too, its at -
http://www.renaxsolutions.com/html/index.php - if you wanna check it out. Your server has to have IonCube support though, you should check with your host (if you are reseller) to see if they support it before you buy it.