I'm working on a project. Our 3D guy has sent us a series of psd images of a police box on a transparent background. However, when I open these psds, the police box is on a black background. He says that's because the transparency is on an alpha channel (please see this screenshot: http://img853.imageshack.us/img853/4531/alphachannelproblem.jpg
However! In all my years of using Photoshop, I've never needed to use alpha channels and so have no idea how to get rid of this black background. Basically, I just need the police box on a transparent or totally white background.
In the channels palette select the alpha channel. Then click the 'load selection' button at the bottom of the palette (a dotted circle). Select the RGB channel again. You may need to invert your selection to select the background (shift+apple+i on a mac, or do it via the Select menu). Press the delete key to remove the selected background.