I have a new Exchange 2010 environment that is coexisting with Exchange 2003. All my 2003 users are perfect, ActiveSync, OWA, Etc. On my 2010 users, everything Except ActiveSync is working. This tells me I have most everything right from SSL to legacy.mydomain.com, etc.
I haven't bothered with Autodiscovery yet, so it's not that. I think it could be some IIS trick to redirect to the ActiveSync folder or something like that which is not right.