Thanks for the suggestions. Let me give an example, perhaps: it is easy to set up a firewall / router box with any distro, but there are dedicated distros like Smoothwall designed for the purpose. Because they are dedicated to doing one thing they tend to do it well and have handy tools built for the job. That's what I'm looking for - a dedicated LAMP distribution that does the job well - and nothing else, necessarily.
But, failing that, I'm thinking Ubuntu server would be a good way to go. It has good community support and security from what I can tell. The reason I usually use Gentoo for servers is because of the over-active forums where I can usually solve something in no time. But I think Ubuntu would be the same, if not better.
I appreciate the suggestions! Keep 'em coming :) I'm not inclined to use Mandriva as a server, but that's just my personal preference.