Since we got asked by a 4,000 user account about moving users into Google Calendar (there's a lesson for you, sinners, you want to get our attention have a 4,000 user migration base), we started looking at moving into Google via iCalendar files.
Most import mechanisms provide the kind of intelligence Geico likes to claim cavemen have, but Google Calendar actually did some checking and gave me the following error message:
But the cool factor is moderated by my noticing that the interest we've heard is decidedly not from the corporate market. I found 3 Critical Reasons Why Hosted Google Apps Won’t Work for Your Business a really interesting read. Though 10 Reasons Enterprises Aren’t Ready to Trust the Cloud does a good job expanding on it.
I also started looking at the issue of making meetings live when we migrate them over. Moving Gmail calendar to Google Apps For your Domain Calendar goes through the single user procedure, but for a few hundred or more users it would be tedious.
I also can't currently get the meetings to come over with guest lists and responses intact.