Extracting the layer transparency into an editable layer mask in Photoshop

This is what I do:

1. Select the layer with transparency
   
2. Click on Menu item Layer, Layer Mask, From Transparency.
   

The transparency will be converted into a layer mask, that you can manipulate from there.

EDIT: in Photoshop CS5 or later.

I have improved Mike's solution (Photoshop CS3):

1. Open/create your image with 'baked in' transparency.
2. Right click on the layer's icon in the Layers window and select Select Pixels.
3. Layer menu -> Duplicate Layer
4. Layer menu -> Merge Visible
5. Repeat steps 3 and 4 several times until there are no semi-transparent pixels. The selection marquee should still remain.
6. Layer menu -> Flatten image. You should see the transparency disappear. The selection marquee should still remain.
7. Right click on the layer in the Layers window and select Layer from Background...
8. Layer menu -> Layer mask -> Reveal Selection
9. Done :-)