As John pointed out, VSTO support for Outlook was announced today. While you could certainly write managed code for Outlook before, this is indeed a big step towards making Outlook a viable platform for managed code. The demos include some very cool ideas....
I've been carrying around a VSTO keychain with the bits on it for a few weeks, but I haven't had the time to do anything more than play around. Hopefully as things settle down a little, I will have the chance to spend a bit more time with this.