From what I can tell, when using the dimension input method you don't actually need the Shift key, just Ctrl. Hold Ctrl, click to start and then click to end the scale. Then when you type in dimensions, it will go in/out in both directions uniformly. In case that didn't make sense, here's what I did to test this:

1) Create a rectangle 2"x2" 2) Pull up 2" so you have a nice cube. 3) Select the top face. 4) Select the scale tool. 5) Hold down Ctrl 6) Perform any random scaling and click to end it 7) Now type in dimensions that you want the scale to be. I chose 4", 6". Hit Enter. This will result in a uniform scale around the center in both directions.

Note: To determine which dimension represents which direction, scale using one of the side handles (ie not the corner ones). Then when you type in the dimensions, the first one will be in the direction of the axis you actually pulled adn the second will go the other way.

Let me know if this was unclear. I'm still a little sleepy :)