iPhone toolbar icons are 24-bit PNGs with an alpha channel and dimensions of about 30 × 30 pixels. The icon’s mask (the parts of the image which aren’t fully transparent) is styled to create the normal and active button states. This works the same way for both Toolbar (blue bar, as in Safari) and Tab Bar (black bar, as in the iPod app). Your icons don't need to contain color or have the gradient style applied to them manually — just make them monochromatic and let iPhone take care of the rest.
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