To quote from the AppExchange listing – "Ever had people post on your Chatter Profile while you're off scuba diving in the Bahamas? Now you can notify colleagues when you're out of the office in Chatter (details about your Bahamas vacation optional)!".
The free Out of Office Notification for Chatter AppExchange package allows users to configure the start and end date/time for their Out-of-Office as well as the OOO message using a simple Custom Object. Every time that a colleague posts on the user's Chatter 'Wall' during the configured Out-of-Office period, an automatic comment is added to the Chatter post with the specified message. The package uses a trigger on the FeedItem object to achieve this and is a great example of how you can use Chatter triggers (introduced in the Spring 11 release) to extend and enhance Chatter.
Download and check out the package (if only to get a quick peek at the Chatter trigger code) and let me know what you think.