A. In a new document, and on a floating layer, select the 'Line Tool (U)' from the Toolbar (It's under the Rectangle Tool).
B. Activate the 'Shape Layers' Options Bar feature for this shape tool.
C. Click and drag to draw a line. (Hold Shift key to create a straight line)
D. Go to 'Window/Styles' menu command to open the Style Palette.
E. On the upper far right of the Styles Palette, select the palette options menu icon. (arrow)
F. From the context menu list choose the style called 'Dotted Strokes'
G. Select from the 42 default dotted stroke presets to be applied to your Line created in step C.

Note: Double click the f symbol to the right of the layer to modify the attached layer effect.

The 'Dotted Strokes' Style Preset also works great on the other Shape Tools (U) such as the Rectangle Tool (U)
for creating dotted borders.

The same style is also available in ImageReady CS.