Our developer Sam just showed me this handy shortcut for quickly changing the size and hardness of the brush tool in Photoshop CS5, and I thought I would share.
With your document open and the brush tool selected, hold down control + alt and right click on the canvas. With control + alt still held down, right click and move your mouse up/down and left right.
When you move the mouse up, you’ll notice the brush get softer and when you move it down, the brush gets harder. The same principle applies when you move the mouse left to right, but instead of changing the hardness, it changes the size. Handy!