Implementing a Load-Balanced Webserver?

Amoeba Protozoa asks: "How do I implement a load-balenced, layer 4 switching web-server? Would it be possible to mix O/Ss? Besides your incoming bandwidth, where do the bottlenecks occur? I would prefer to use Apache, Linux or BSD, and be able to utilize mod_perl or PHP to access a shared MySQL database. I would like to make this setup as scalable as possible."