Nope-- you can't just move the database around. It's too bad, but that's just the way the product is designed / implemented.
Your best bet is to ExMerge out the data from the source server, and import it into the Exchange 2007 server.
More background on ExMerge: http://support.microsoft.com/kb/174197
Bear in mind that replies to the imported messages may fail and modifications to old calendar items may not work right because the X500 addresses from the original Exchange install won't be assigned to the mailboxes in the new Exchange install. There are some details about that problem here: http://blogs.technet.com/sbs/archive/2009/05/21/cannot-reply-to-old-emails-or-modify-old-calendar-items-after-pst-mail-migration.aspx
Basically, you need the legacyExchangeDN from the old Exchange install to be added to the new recipients as an X500 format proxyAddress entry. It's not too bad to do, once you know what to do.