In my opinion Clustering is between the Administrative Server and the backend database. It has no revelence with the HTTP server being used. In case you use the Edge server for load balancing between your HTTP servers even then there may not be any performance issues. But honestly this is my opinion lets see what others have to say.
IBM Http Server is a built on the Apache Web Server. The Web Server plug-in redirects the request to the AppServer, which handles all the load-balancing issues. The WebServer hardly has any role to play in load-balancing.
In such a case, I don't see a problem in handling load-balancing issues using Apache. Hope it helps. Cheers, pd
subject: Compatibility of Apache Web Server with Websphere Application Server