The developer of myBilling.info I believe is a member of this forum (Intrahost). He's also arranged for a myBilling.info forum be setup at HostingChat.net... you could check there for more info about it.
I personally am not interested in a proprietary solution. I will begin using and developing for <a href='http://www.gnuenterprise.org' target='_blank'>http://www.gnuenterprise.org</a> as it matures.
Is it too early (or too late) for a newbie to start talking about her company? We are about to launch a billing software that manages a good deal more than just billing.