No experience here txbakers, simply my opinion... If memory serves you are selling hosting on your operational IIS server. Based on threads here and at other forums, IIS does not seem to be as stable as other web servers. For this reason alone, I would avoid anything that offered a potential conflict with IIS. (I'm not bashing M$ here, just stating my opinion/impression.) Unless someone you trust has first hand experience running both servers and assures you it can be done without conflicts, don't! And let's hope someone offers some real info based on experience soon instead of just my lousy opinion! :)
I downloaded the JRun app server from Macromedia (development edition) and installed it on my test server . It does integrate with IIS, and after some configuration will route jsp pages directly to that app server.
So, I'm not running two web servers, just two app servers. Which does make sense to me. ASP needs to be 'registered' with the web server even though ASP is embedded in IIS. Perl, which is similar to an application server has to be registered, and so do the java dlls for JSP.
But thanks for your concern. this is all taking place on my test server - not the production server.