I just finished trying to do this a couple of days ago, and gave up. When you sync with Google calendar, all of your information has to originate in the Google calendar. Once you've got your CalDAV info entered into your iPad (or iphone), its native app then reads the info off of the Google calendar and syncs with it. It doesn't go both ways as far as I could tell. That is, you can't enter stuff into the native app and have it sync to your Google calendar.
I finally gave up because trying to use the Google calendar was a total pain. It doesn't even have a copy/paste option (which I use to enter on-call jobs that I have at unpredictable times, but which are similar enough that a copy/paste function would be awesome). Plus, what's the point of entering everything into the Google calendar and then using the native app to read the events? You'd just end up using the Google calendar, which I found much less readable on the iPad than the native app.
As far as I could tell I need to have a Mobile Me account if I want that functionality, but it's not worth it to me. So, I do something similar to what one of the other members suggested above: I enter my info on the iMac, then sync the iPad. I'm not really satisfied with it, but live with it.
If I'm wrong about this, somebody please correct me. I won't be offended if I find out I can do it another way. I'd love to know.