Hi Andrew... [excited]
Not too difficult to do this actually. Just gotta be creative with a few tools. 
Keep in mind that this is but one way to do this. I know many more, but this is quick/easy to explain. Only 4 easy steps! 
1) Make sure you photo is on a layer by itself; and not the default bg layer of your document. It should also have at least 20-30 pixels of empty area around it, so we have room to work. (pic 1)
2) Add a new layer below the photo layer. Use the LASSO selection tool and manually draw out a jagged, sloppy looking selection around the photo. Fill the selection with black. Deselect. (pics 2 & 3)
3) Grab the BLUR tool, set it to 35-50%, and manually blur some spots around the edges. This is user dependant. Use your own taste. Don't go overboard though. Be subtle. Just some selective spots -- 2 maximum on each side. (pic 4)
4) Apply the FILTER>Other>Minimum filter again and set it to between 1-3 pixels; depending on what looks good to you. (pic 4)
And that should do it.
TIP:if you'd like to thin out the border, use the "FILTER>Other>Maximum" filter.
A slight alteration to this method would involve selecting the border shape (step 2) and adding a Layer Mask to the layer. Then applying 1 or 2 specific edge effecting filters to the mask -- like "Spatter".
Experiment with this too if you like.
The added bonus of using the alternative method is you can experiment with specific filters and not have to redo the whole main border image if something gets mucked up. Just reselect the border shape, invert the selection, and fill it with white; which rests the shape on the mask. So you can just start over with your experimenting.
Hope that helps. [slick]
