A relatively sleepless weekend, and not in a good way. Here's an interesting tidbit, the common unix accounts such as postfix, mysql, www, and others, are now prefaced by an underscore character in OS X 10.5. Why, I have no idea, but when configuring scripts, make sure to change the usernames.
The switch to Apache2 for the most part went pretty well, it took a bit of trial and error to get my virtual domains working, but once I went back and poured over the sample configs, it all started to make sense. Just copying and pasting relevant bits from my old config files was not the way to go. :)
Minor issues compiling the DBD::mysql module, I had to use a slightly older version and it worked fine.
Courier-IMAP is what drove me absolutely nuts, I finally thought I had it all going, and then discovered that the auth module was throwing errors in the log, and I'm still trying to track that down. Also, despite attempting to migrate my older Courier settings, this apparently didn't happen, best thing may be to build it all up from scratch.