OK, I run a medium-large web forum and have postfix set up to handle user varification emails, new post notifications, and stuff like that.

I want to use google aps to provide @mydomain addresses for my admins. Is there any way to do this without interfering with the operation of postfix for sending the automated mails, or will changing the MX records to use google aps cause mail coming from postfix @mydomain?

Nope MX records are for incoming mail, if you edit them postfix will be fine.

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Make sure you don't have postfix configured as the destination for @mydomain, or else your own users may not receive your notification emails.

If you set up an SPF record, make sure it has your postfix server ip called out in addition to the google-recommended setting.