This has been a long standing question - when do we switch the mail system over to Google?
There are several problems, the main one is to do with mailing lists.
We use mailing lists to send mail to groups of people - staff, 7th grade, mathematics department, etc. Our system allows us to write rules, the most important one being "do not allow anyone from outside the primary domain to send a mail to the mailing list".
This prevents the reply to all fiasco from outside or the spammers from getting hold of a group of (or indeed all) people.
GMail in GApps cannot do this (Lauren from Google confirmed this although said that they might do this in the future).
So we will have to revert to a dual system....