In a recent post to his blog, Brian Moon talks about the experience he's had with using PHP on Apache 2 with a threaded MPM at Dealnews.com.

Well, first, what is an MPM? It stands for Multi-Processing Module. [...] The most commonly used threaded MPM is the Worker MPM. In this MPM, you have several processes that run multiple threads within it. This is the one I will be talking about.

He goes on to describe the "huge memory savings" that using the module got them - using the worker process to increase the child capacity of the Apache instance. He recommends keeping it simple when compiling a server to work with the worker functionality and mentions how it can also help with serving static pages right along side the dynamic ones (without a second server!).

Check out the post for a few more tips.