After what appeared to be my straight-forward upgrade, I found my Address Book reported no content and then froze -- requiring a Force Quit. When I launched Mail, initially it would open looking normal, but then it, too, would freeze. (Note: Once the problem with Address Book was solved, Mail was then OK.)
For me, after several hours working with AppleCare technicians, the cause turned out to be that my Address Book under v10.5.8 included a Smart Group with a negative condition (i.e., "Card is not a member of any group"). This was the only Smart Group I had created. The AppleCare technician ultimately found a report that such Smart Groups are not being handled properly during the upgrade.
I had done a SuperDuper! clone (on an external hard drive) of my MacBook Pro immediately before doing the upgrade. With the AppleCare technician pointing the way, I rebooted into Leopard from that external hard drive, launched Address Book, and deleted the Smart Group. I then copied the ~/Library » Application Support » AddressBook folder from the external hard drive (after adjusting Permissions, as needed) to the comparable location on my MacBook Pro once it was rebooted from its internal hard drive in Snow Leopard. But I wasn't out of the woods quite yet. Address Book would now launch, but it still claimed no cards existed. (There were, however, the proper 5,000+ cards in the Metadata folder within that AddressBook folder.)
While having deleted the Smart Group within Address Book application under Leopard before copying that AddressBook folder was expected to have deleted all remnants of that group, the technician wondered whether that was, indeed, the case. He then had me search that Metadata folder within that AddressBook folder for any Smart Group file by typing "smart" in the search oval as that file's long cryptic name, would include "smart"--and such a file was found. (In my case, it ended with ABSmartGroup.abcdg.) I deleted that file, restarted the computer, and Address Book was fine. And as noted above, when I then tried Mail, it, too, was fine.
Along the way of trying other things that didn't work (e.g., transferring an Address Book Archive made under Leopard and importing it under Snow Leopard; doing Export Group vCards under Leopard and importing them under Snow Leopard), we found importing Group vCards under Snow Leopard doesn't preserve the Group identification and while, then, hoping to use Snow Leopard's Address Book's "Last Import" Group to identify the last import, we found that Group doesn't properly report just the last import. (The AppleCare technician is reporting those issues within Apple.)
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090830161634673