My iPhone syncs to Google Calendar through Exchange/ActiveSync, I imagine that will still be an option.

Edit: I forgot they're turning off ActiveSync for free users later this year

Pure speculation, but I would assume Apple will be a "whitelisted developer".

Based on Google's previous app releases (YouTube, Gmail, Maps, ...) I wouldn't be surprised to see a native Google Calendar app instead.

It's funny because while I'm bound to my gmail account by my email address, I use google calendars only because of inertia. I would think that it's a huge benefit to google to know about all of my appointments. The day I can't use google's calendar with Mail.app, either on iOS or OSX, is the day I migrate my calendars to iCloud. Google cant be dumb enough to think that iOS users will use separate calendar apps for their exchange and gmail calendars, can they?

So what's next - no IMAP for gmail? And then force everyone to use a custom gmail app?

If that's the case, then I'm leaving Google calendar. Apple doesn't get to pick which calendar app I use, let alone Google.