GammaSlamma a brilliant little app that lets you batch remove Gamma information from PNGs.
By simply dragging a PNG file onto the GammaSlamma icon, a new version will be created automatically with all that nasty Gamma information removed, making PNGs perfect for flash without having to hassle with levels and stripping manually.
["May 30th, 2011: Unfortunately 4 years after its inception it is increasingly 404ing. So here a link to the DMG in case you need to download it: GammaSlamma 1.1"
CW 9/05/2012 Update: I pointed the installer link to my server (instead of Andreas Wacker's blog) because my file still works.]