I would use the quickmask in photoshop, then use a semi soft brush to make the selection and save it to an alpha.
Quickmask mode is at the bottom of the toolbar. Make sure your foreground and background colors are 0,0,0 and 255, 255, 255 (pure black and pure white). You can toggle between them with the x key and this will add or subtract your selection.
Paintshop Pro and Gimp have some nice masking tools that use edge detection but in this case where the fence is so blurry your probably better off doing it by hand.