I'm using cycles to render transparency around some leaves, but the actual transparency has a slight grey tint. I read some other place on google to increase the Transparency under Light Paths in the Render Panel, but it doesn't work for me.
Also don´t forget to check the ALPHA on Photoshop or GIMP, by placing a full black fill on your image as a lower layer. Delete (clean, CLAMP) your white alpha as white as it can get. Then, reload your image in Blender. This should cover it.