OK, for those of you stuck with double (or triple) calendars, of which one is your Outlook/Exchange 2007 calendar, here is a short howto on including/integrating your Google Apps calendar (or any other Google Calendar) into Outlook 2007.
First step is to ensure your Google Apps domain allows sharing outside of your domain:
You can find these settings under 'Manage your domain' -> Service Settings -> Calendar.
Now what we are after is the 'private ICAL URL' of your Google calendar. This should be reachable in yoru Google calendar via Settings -> Calendar (tab) and then selecting your main calendar (could work on the others too, if you own them):
If you see this right away, you are lucky :-). In most cases, the Private URL is not visible. You can enable it by sharing it to any e-mail address outside of your Google Apps domain. Perhaps the e-mail address associated with the Outlook 2007 is a good idea ;-). You can do this by selecting 'change sharing settings' (see the screenshot above). Returing to the calendar settings should now reveal the private address. Click on the green ICAL icon to show the URL and copy this:
Now you could add this URL in Outlook via Tools -> Account Settings -> Internet Calendars (Tab) and then pasting it, but if the URL starts with https://, this doesn't work. Paste the URL. Instead, change the https:// bit with webcals:// and paste it into your browser. This will open Outlook (if you permit) and ask you to subscribe to the calendar:
Hope this works for you, it worked for me :-). It took me about three different sources on the internet, with various pieces of the puzzle:
http://www.howtogeek.com/howto/microsoft-office/view-your-google-calendar-in-outlook-2007/ for the initial howto, http://markmail.org/message/ud3ad5ak3wekr3go#query:google%20apps%20private%20calendar%20URL+page:1+mid:ud3ad5ak3wekr3go+state:results for the trick on getting the private URL enabled in Google Apps and last but not least http://ryanmaizel.com/2009/07/03/google-apps-outlook-internet-calendars-and-ssl/ for making Outlook 2007 understand my https encrypted ICAL calendar.