Valve: Steam Offline Mode "Designed to be Indefinite"

A response from Valve to a thread about offline mode on Steam indicates that the way offline mode currently requires users to go online periodically to allow them to play their Steam games without being connected to the Internet is a bug, and not working as intended. He does not offer a timeframe for when this might be fixed, but clearly states the requirement to go online once your game is authenticated is a "bug" (thanks Creston). Here's word:
This is not actually true - Offline Mode is designed to be indefinite. You can't access any of Steam's online features such as friends lists or saved game synchronization, of course, but the client should allow you to run in Offline Mode for as long as you like.

That said, there are many components involved in Offline Mode, and some of them have known issues and bugs which we are continually working to improve. We're aware that it doesn't always work as flawlessly as we want it to, but please keep reporting bugs with Offline Mode. It is not broken 'by design'.