Ken, are you referring to the interface elements (the titles of all the text fields, etc) or to the content being authored by users of the CMS?
Putting 2 labels on each interface element would clutter things up a lot and would be difficult to do well. It would probably be easier to have an quick way of switching between a few different languages.
Sam&Tim, a possibility to provide the same content in different languages is very important f.e. in Germany. Usually for a "professional cooperate" website, its critical to have at least rudimentary versions of English ( and often france too ).
I am not so much looking at the interface, but the ability to place multiple languages on the displayed page for the end user. For instance, to create something like an English/Chinese or English/Maori dictionary would be an extreme example. If alt/title tags could also display multiple languages that would be great, but not the major "want" for me.