Start out by making a blank document.
First of all, draw a Rounded Rectangle with 25px Radius and Resterize it (right click the rounded rectangle layer and click Rasterize Layer).

Select the layer and double click on it in order to open Layer Style dialog box and apply following settings.

Your result should be something like this.

Now select Background layer then press Ctrl+A and fill background with black.

Now we add a small highlight to button to give it more web 2.0ish style.
Select Rectangle Tool and make a selection as shown in image below.

Go to Select > Modify > Smooth Selection.

Result:

On a new layer add a smooth white to transparent gradient.

Result:

The final result.
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