got this answer (which works!), through the Apple Servicedesk in The Netherlands. Too bad I experienced this problem, but fortunately it was solved real fast:
With upgrade to leopard it is possible that two files become corrupt:
messagerules.plist
messagerules.plist.backup
These files are located in de /Library/Mail folder.
Shut down your Mail application
Remove these to files (by placing them on your Desktop)
Restart Mail and Presto, all messages are displayed!