The only issue with that is that you require an SMS gateway. Not entirely sure if Asterisk can provide that, if it can then that might be possible via the zimlet when the two products talk nicely to each other soon.
One question I have with that is would it be condusive to collaboration? Remember Zimbra is a Collaboration Suite primarily, it just so happens that messaging is a big part in the whole collaboration sphere. I would rather ZCS be a bit light so that it does the functions it was designed to do well, rather than do a load of functions poorly.
Just my thruppence of opinion
Andy