BetterTouchTool lets you do this: http://boastr.net/ (
it's free and quite handy!)
UPDATE: Since 2016 BetterTouchTool is not free anymore.
In BTT's prefs, click on the "Action Settings (window stuff etc)" button.
Then select the "Window Moving and Resizing" tab.
The bottom pane will have 3 options, choose whichever you like:
I selected all three: move, resize, and FN.
When I hold FN+CTRL the focused window will move when I slide around on the trackpad.
When I hold FN+OPTION the focused window will resize when I slide around on the trackpad.
I tested this on my macbook pro, mac os x 10.6.5.
Note that you may get this message first time you try to actually move or resize a window:
Click "Launch it for me!", then check the box for "Enable access for assistive devices" - you're in business.