In this post, I mentioned that I wished that the lack of IMAP support was preventing me from switching to Google Apps for Your Domain. Last night, I was thinking about this, and I relalized that I already have an IMAP server. I just need to get the mail to the server.
Fetchmail can be used to deliver mail from pop servers to other servers. This page has a script that can be used to start fetchmail when the computer starts. This page describes the how to setup fetchmail to check Gmail through fetchmail.
Now I can use fetchmail to deliver my mail to my server, so I can use my seive filters on the IMAP server. But for for the other accounts, I will probably move them to Gmail, and just use POP3 access.