It is written here http://wicketstuff.org/confluence/display/STUFFWIKI/wicket-contrib-datepicker that 'Package names have been kept the same, so this is a 100% backwards-compatible direct drop-in replacement for the old component.'.
But really the package names are different. Wicket 1.3.3 version has packages started from org.apache.wicket... but the wicket-contrib-datepicker-1.2.zip has packages started from wicket....
So it means that it is not drop-in replacement.
Are you going to support old wicket calendar component for new versions of wicket?