cradz,
unfortunately we're still working to get things finalized but the end result is this...
plesk 6.5 does a strange thing with the file paths that doesn't allow you to use "include virtual" and the mapath feature had some trouble. this is a problem for us b/c all our code was written using these 2 things so changing them to "include file" and absolute file paths can't work for us - this might not be an issue for what you're doing in which case i would expect it to all work pretty smooth. our solution involves running one server with plesk and modernbill that points to another server running only IIS that houses the sites using a more normal file path structure. we're having some issue with frontpage extensions causing our automated process to be less automated but as of right now it appears we are fixing that up a bit.
depending on whether you use includes or how your file paths are setup, i'd say to search the board for more info on this to make sure it won't be a problem for you. modernbill is pretty slick and fairly easy to configure (beyond what the host configured for us). i haven't tested the actual account creation through modernbill but i'm assuming it all works.
i think most of our problems are somewhat specific to us but be aware that frontpage extensions might not work with the skeleton file as far as holding their permissions.
anywho - we are going to try and setup a few clients tonight and see how it goes.
some cool features of modernbill include the checking and registering of domains during the setup process and automatic setting of dns info - all in all it's pretty slick and worth looking at. by monday we should have a pretty solid understanding of how well this works so check back then.