Applications can contribute messages to the message area of the status bar. Be sure not abuse the status bar; it is for status only. Ensure that important actions are available from the menu bar.
Put a message in the status bar when the message is informational only, for example, a confirmation that a person has been added to your Contacts list.
Pop up a dialog box when the message informs users of lost data or potential lost data, or some other failed operation, such as a mail compaction failure.