I haven't tested
this yet, but Make a Delivery
sounds like a handy
little Automator action that will create a .dmg file from the Finder items you have selected, then attach that .dmg to
an email. One quirky thing, however, is that you have to move a resource folder into your Applications directory; it
isn't just an action that you can use or turn into a right-click Finder plugin.
In any case, if you prefer
an easy way to create .dmg's for emailing (instead of zip archives), Make a Delivery
might be just what you need.