http://www.mediafire.com/download/4as7iertgc8oys2/Birdie's_Scene_Switcher.zip
Unzip the file. Start up OBS and Brood War (it should work for other streaming programs as well, except that xsplit for example has a changing window title. I'll try find another way to get the window handle besides using the window title). Change the hotkeys to what you use for your scenes in settings.ini, and you may need to change the window title too (at time of release the OBS window title is Open Broadcaster Software v0.522b but it could have changed).
Right-click BirdieSceneSwitcher.exe and Run as Administrator. Now when you enter a lobby or a game, it should change scene to whatever your OBS scene is set to, and when you leave a game it will change back.
It's currently quite untested so if it doesn't work for you, please post in this thread with your OBS version, operating system, and whether you're 32 bit or 64 bit, and I'll try to fix any problems.
It doesn't support modifiers for the hotkeys (Shift, Alt, Ctrl).