Anders Fjeldstad posted a neat JS shim on GitHub that adds in the ability to make CRM 2013 notifications in CRM 2011.
You can wrap it in a function that calls during your form’s CRM onLoad event. That’s it! Then you can make notifications appears easily:
Xrm.Page.ui.setFormNotification('This is an error',1,101); //Error (signified by the 1, with messageId 101)
Xrm.Page.ui.setFormNotification('This is a warning',2,102); //Warning (signified by the 2, with messageId 102)
Xrm.Page.ui.setFormNotification('This is informational',3,103); //Informational message (signified by the 3, with messageId 103)
Removing a notification is the same as in CRM 2013:
Xrm.Page.ui.clearFormNotification(101); //this clears messageId 101
Xrm.Page.ui.clearFormNotification(102); //this clears messageId 102
Xrm.Page.ui.clearFormNotification(103); //this clears messageId 103