I had a previous question that was answered regarding Free/Busy showing the hash marks for users on Exchange 2010 looking at user calendars on Exchange 2003. We are co-existing w/Exchange 2003 while we migrate users off.
Just 2 servers. No Edge Transport server. Both servers can send/receive to the internet and each other.
Once I setup the Replication between both servers, the hash marks have gone away when users look at user calendars.
HOWEVER - they do not see any item in the user calendars. Like I said, the hash marks are gone but there is no calendar data. This is not true for Exchange 2003 users looking at Exchang 2010 user calendars - that works fine.
Also, Exchange 2010 users can see calendar resources on the Exchange 2003 server just fine, it's only the user calendars that are not showing any data.