With BetterTouchTool, you can create a Magic Mouse action to middle-click, such as this:
For trackpad users, BTT gives you the option to perform a 'special' middle click designed for CAD users (accessible through Advanced → Action Settings → Settings for Predefined Actions → Use special middle click mode).
This allows people who work with CAD software etc. to perform a real middleclick using the trackpad. Use predefined action middleclick in combination with "three finger click" gesture. Click with 3 fingers then release two of them and move the remaining finger to drag the view around.