We installed Exchange 2007 a few months ago, and against all advice, installed it into an environment that had both Windows 2000 and Windows 2003 DCs. A lot of people think it couldn't be done, but there was docs out there and all went relatively smoothly.
Out of 100 users, about 10 of them couldn't connect through Outlook 2002, and I needed to set "DS Server" in 'HKCU\Software\Microsoft\Exchange\Exchange Provider' to the Win2k3 GC. That woked well.
My own personal computer which is running Outlook 2007 did not require such a fix until recently. I uninstalled a bunch of stuff and all of a sudden I got an error message that the Exchange Server was unavailable. I modified the registry but it didn't seem to do anything. Then I ran a network analyzer (ethereal) and noticed that it was querying the wrong GC for the info. So it apparently isn't using the 'DS Server' regsitry value.
I've verified that the registry key is entered correctly and I've rebooted the computer several times.
Does anyone have any idea why outlook would not be using the GC I tell it to in the DS Server setting?