applications for all form factors can accept two images for the icon: a regular icon and a hover icon, which is displayed when the user hovers over the app with the cursor.
BlackBerry 6: "For application icons, create an icon file that does not exceed 68-by-68 pixels. This size includes the canvas. Keep the weight of icons inside a 49-by-49 pixels space. Smaller elements can extend beyond this size but icons should not exceed 55-by-55 pixels. If the icon within the canvas exceeds this size, it could be scaled down and upset the line weights and gradients."