A one word answer to this question is: REMOVED.
As we all know that System Attendant mailbox was used to publish the free/busy info and the link monitoring service in earlier versions of Exchange. In Exchange 2010 it the mailbox object for System Attendant has been removed. Yet, you can locate the directory object for System Attendant in Active Directory. Per a post published by Dave, the mailbox object for system attendant has been removed but can be seen in AD. This change was done due to the new high availability architecture of Exchange 2010. They observed problems in failovers while having the mailbox object for System Attendant.
How will this affect:
If you are running an application that uses System Attendant to connect to the servers/databases you will still be able to connect but not logon. That means you wont be able to open the system attendant mailbox anymore as the actual related mailbox doesn’t exist.
The way around it:
1. Use the per mailbox database System mailbox (See Additional Resources for more information about this mailbox).
2. Change your application to rely on a user created account and mailbox.
This has been described pretty well at he System Attendant mailbox has been removed from Exchange 2010
And do read the additional resource section to understand the facts about special mailboxes in Exchange.