So what you do is use the Magic Wand tool and Shift-click to progressively create a selection area of either the foreground or the background. Keep ready on the Undo key if you accidentally select too much. Concentrate on the border between the foreground and background, dont worry about the outlying areas for now.
You can play with the tolerance (up on the toolbar) to determine how wide a range of colors that will be selected. There is pretty good color sepraration in your photo, the only problems you'll have are the folds in the curtain and the dark hair. Once you have got as close as you can with the magic wand, then start refining the selection with the Lasso tool, zoom in and use Shift to ADD to the selection and Shift to SUBTRACT from the selection. Take your time to get as accurate as you need to be. Use the Lasso tool to clean up the stray selections in outlying areas.
Now Save your selection before you go any further, so you can come back to it later if you need to.
Unless you have done a pixel-accurate job, then Choose Select: Feather and feather the selection by a very small amount. Use Select Inverse if you have to select the foreground, Copy, and Paste it into a new Layer. Turn off the Background layer and see how good you got it. Now you can create a new layer behind the foreground and slide in your new background.
EDIT: Skunk, you're too danged fast.
MacAz, if you want a less jagged outline, you can use the same trick: Use the Magic Wand on the white area with a small Tolerance, feather by one pixel, and hit Delete. This'll fade the edge a bit.
