My parents' domain is registered with Google Apps (standard edition). Currently, the only service they are using is email (which they access via IMAP in Outlook). I'd like to register this same domain with a hosted Exchange service so that they can sync their Exchange calendar, tasks, and contacts with Outlook as well. I'd like to keep their email on Google for a couple of reasons. Is this possible?
Not to my knowledge. A single mailbox needs to exist on one service. You could have separate LDAP and Calender accounts on other machines, but then you loose the integration you'd normally get.
Ignoring that Exchange is going to expect the account to have full functionality.
At least with the Premier and Education editions of Google Apps you can use ActiveSync and the Outlook connector to access everything within Google Apps. This should negate the need for Exchange for the most part.
I am not sure if this will cover notes/tasks, but there are certainly other ways to do those if it does not.
There should be more information in your GoogleApps control panel regarding both features. If it is not in the standard version I can take a close look in the Education version which is what I maintain for work. We have both working for one group of users. We do run Exchange for another group of users however. (Students and Faculty are on Google Apps, Staff and Administrative users are on Exchange)