When sending a meeting invitation (via an Exchange account) from an iPhone with iPhone OS 3.0 or later to someone using who uses an email application (such as Microsoft Outlook 2007) and a different Exchange Server than the sender, the recipient may accept the meeting invitation, yet appear to not be marked as having done so on the iPhone which sent the invitation.
In other words, this issue is not likely to occur if the sender and recipient are both within a company that uses a single Exchange server, but may occur when someone from one company sends an invitation to someone from a different company.