UITabBar icons for iPhone

I finally found out how to get a normal image to a alphatransparent icon for the iPhone UITabBar without colors but with different alpha transparencies. I do it in Photoshop like that:

1. Image -> Mode -> Grayscale
2. "Discard color information" -> Discard
3. Mark all [cmd A]
4. Copy [cmd C]
5. Layer -> Layer Mask -> Reveal all
6. In "Chanels" (Window -> Chanels) show the "Layer 0 Mask" (click on the left)
7. Paste [cmd V]
8. Image -> Adjustment -> Invert [cmd I]
9. Save for web [cmd alt shift S] as PNG 24

I found it on a website but I can't remember which one, so I'm sorry no back-link today. But if I don't jot it down here I'll never remember how to do that.