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File>save for web>select gif from the drop down and check transparency
That's pretty much it. You could also start on a white background, make whatever you need then erase the bg or delete that layer and save as a transparent gif again...
You might also want to assign a matte color to the image in the Save for Web dialog. You want a matte that is either the same color as the background the image will be placed on or an average sampling if it's on a background that is a pattern.