I've been running IE7 Beta 2 for a couple of weeks now, and I have to say I like it more the more I use it (but not in a weird way you understand).
So anyway, today I thought I'd visit the Google Calendar site, since I'm interested in the API...and what do I see but a message saying "Sorry, Google Calendar doesn't currently support your browser". Clicking the Cancel option to ignore the message I soon found that Google Calendar appears to support my browser quite happily, so it looks like a case of unsophisticated handling of the user agent string.
But wait, there's more: when I came to write this post I found myself confronted not with the usual nifty editing tool but a simple Textarea control. Since I have reason to believe that IE7 Beta 2 has no particular trouble with the contenteditable attribute (I was trying it a few days ago), it looks like Community Server has the same problem as Google
(I'm reminded of ASP.NET 1.x, which would happily class any non-Microsoft browser as "downlevel"...but Microsoft at least have learned).
Update: I was doing Community Server an injustice
I just tried a post from the version of Community Server 2.0 that I have installed on my home PC for testing against, and it presents a rich text editing interface that works fine under IE7 Beta 2...so it appears that the problem is with CS 1.x
So at least they fixed it for later versions.