I finally found the answer to the above by digging around in the WIKI. Webmail works via webmail.dreamhost.com.
Now, however, we're on to the business of trying to migrate the contents of the old mailboxes over to Google and I can't find how to access the old mailboxes via IMAP now that mail.domain.com now points to Google.[hr]
I checked the headers of some emails sent yesterday trying to find the server name and I found
homiemail-a10.g.dreamhost.com (caiajhbdccah.dreamhost.com [22.214.171.124]
Which of those would I use for the IMAP migration/import tool?