There is an interesting operation option in SharePoint called “Quiesce Farm” in MOSS (Section Global Configuration –> Operation of Central Admin) and in SharePoint 2010 via stsadm -o quiescefarm -maxduration n command.
Please note that not all features in your portal environment are affected when you Quiesce your farm. Quiesce doesn’t mean the same user online can’t do anything or a new user can’t access the SharePoint site
There are two features that make use of Quiescing:
Microsoft Office InfoPath Forms Services (IPFS) and
Microsoft Office Excel Services.
Many other features and operations do not need to use Quisecing because they do not have long running sessions where users enter data over multiple server requests without having to save information. For instance, when editing and item in a SharePoint list, SharePoint handles that in a single transaction to the database, storing the information.
However, in IPFS, a form filling session may require several communications with a server as the form posts back for server-side data processing for operations such as view switching. Data from the session is usually not saved until the very end when a user submits or saves the form that he or she is filling.
If an administrator takes the farm offline while some users were already filling it out, the users would lose all of the data accumulated so far in their session.
Therefore, if an administrator is going to bring the farm offline, in order to preserve their customer data, they would first quiesce the farm. This prevents new requests from coming in to start filling out new forms, but it allows existing form filling sessions to continue. When the sessions are all completed, or when an administrator-specified time elapses, the farm enters the quiesced state where no new requests are accepted. The farm can then be safely taken offline at this point without causing any data loss for users.
Hope it helps!