An Update!
After much toil (and reading / re-reading all the emails about this on the list) I think I have it working! Huzzah.
Exim is happily handing mail to procmail, which happily files it as it should. I am using my ISP's smtp server as the smarthost, so mail is all going out as it should, and doesn't seem to being blocked by any servers so far (as it was when my dynamic-ip-server was the smtp. Mutt knows how to read my SMTP folders. Squirrelmail knows how to send from different identities (a very cool feature...anyone know if Mutt can do that?)
Still adding to the procmail as different types of messages come in...and haven't tackled spamassassin yet. But thats next. Having it this far feels great!
Thanks for all the help on this everyone.
Matt