Pidgin Full Screen Busy Plugin
This (Windows-only) plugin for Pidgin will detect a full-screen application and automatically switch to a DND (or any saved) status.
To install, download and copy fsb.dll in to your plugins folder (I'll make a proper installer soon, I promise). After enabling the plugin, configuration will allow selecting a saved status.
Changelog and Download
2010-11-02 (1.3): Fixed issues with substatus corruption and defaulting to AWAY instead of saved status
This plugin isn't currently maintained, due to pidgin's horrible attitude to password safety. While passwords are stored in plaintext, I can only recommend using another IM client (such as Trillian).