this is a strange move by ubuntu/debian, dash is a stripped down posix shell. with edgy ubuntu has done a few things to try and really make it much faster but has some pretty radical changes in it - personally I would avoid trying to run zimbra on edgy until it's officially supported unless you're just playing.
if you want to reset the main symlink, do
rm /bin/sh
ln -s /bin/bash /bin/sh |