Are you using Exchange 2007. if you are make sure that the calendar information is being put into a public folder so that Outlook 2003 can access it
It would be helpful to have more information. You do not tell us your Exchange version or if you are in an mixed Exchange version environment. You say half are on 2003 and half are on 2007, but do you mean Outlook 2003 and 2007 or Exchange 2003 and 2007? If mixed Exchange environment you need to mount a Public Folder store on the Exchange mailbox server to communicate with the Exchange 2003 calendar info and you have to make sure you are replicating the schedulerplus free/busy folder to the Exchange 2007 public store. You can do this easily using PFDAVADmin, which you install on your pc, not the Exchange server. Please provide more details. Thanks.