To use straight IMAP and not the GMail Plugin:
1. Delete your GMail account from BIS.
2. Go into your GMail account, under Labs, and enable "Advanced IMAP". Then go under settings, Forwarding & POP/IMAP, and choose disable pop, enable IMAP, do not expunge messages, archive message when expunged. Then go to labels and check "Show in IMAP" for everything but trash.
3. From BIS, add the email address
[email protected] with whatever password you want. It'll fail (obviously) and ask you to confirm your info. Click on the link at the top for manually entering settings and enter your gmail address for your username, your password, and imap.gmail.com for the imap server. Enable sync deleted items.
4. In the phone email settings, set it to delete items on server and handheld.
Now your messages will get delivered to your phone like any other e-mail address (unthreaded and without any of the options to add star, set labels, etc). If you delete them off of your phone, they will get archived in gmail. I could be mistaken, but if you archive messages from your gmail account, they do seem to disappear off of your phone eventually (hours later).
I don't know if your old messages will get deleted from the Messages folder. Probably not. You can always go through and delete them if it bothers you.