Step 1: Create a new 400x400 pixel document, and Edit > Fill the background layer with a dark grey.  Create a new blank transparent layer on top and then, using either the pen tool or the rectangular marquee tool, create an object resembling the one on the left.  The exact shape of the object is entirely up to you - it just has to resemble your proposed interface.  Minor imperfections should be ignored too - the reasons for this will become apparent later...

Step 2: If you have a pen-path object, you should now rasterize it. Then you'll need to CTRL+click the layer, to select its contents. Make sure that you have black and white as foreground and background colours. Using your selection and the gradient tool, fill it with a gradient that looks a little something like the one on the left.   Don't worry if it doesn't look perfect - this will all add to the effect!

Step 3: Using rectangular selections and gradients similar to those in step 2, add a few random bars to your creation.

NB. You might want to play with image > adjust > levels to increase the contrast as necessary.

At this point you might want to add some more random boxes.  With this complete, we'll progress onto the wires & mesh:

Step 4: Create a new transparent layer on top of the others, and call it mesh. Now create a new shape like the one on the left  and Edit > Fill it with black.  CTRL+click the layer and select Edit > Stroke from the menu. Give the object a 2px stroke outline, and then lower the opacity of the whole mesh layer to 45%.

Step 5: Now draw some quick black 'wires' onto another new layer, with a 5px thick brush set to 100% opacity. Again, there is no need for them to be perfect.  In fact, that this point the image usually looks pretty bad anyway.

Step 6: Create another new layer, and make a circular selection on top of your existing design. Fill this with the gradient tool using a gradient of your choice, and then drop the opacity of the layer down to 50%.

Step 7: Click on the 'brushes' tab within Photoshop and duplicate the same settings as those on the left. The brush tip you select should be small and scatter well.

NB. Please note that the 'brushes' tab may be in different places depending on your version of Photoshop and OS. If in doubt, consult your manual.

Step 8: Click the Shape Dynamics checkbox in the brush dialog, and enter the settings on the left into the interface.

Step 9: Repeat the above process, but this time with the Scattering section of the brush dialog, using the settings opposite.  Once this is done you can leave the brushes dialog.

Step 10:  Create a new transparent layer on top of all the others. Using your newly-created grunge brush, add some grunge by drawing around the top left edge with light grey, and then again with dark grey.  After a little time it should look like the image on the left.

Step 11: Now comes the easy part!  Marge all your layers together except for the background layer.  This will leave you with a BG layer, and an object layer. Now, click on the object layer, and use Edit > Transform > Scale to shrink your object down to a very small size.   If you do this correctly, you should now have a small piece of a grungy interface.

Step 12: Well, now that you have the hang of this technique, repeat as much as desired to create your ideal interface. You just need to add some lines and a box. Make your lines grungy by using the same sort of settings on your brush as steps 7-9, but with the eraser tool. Finally, add a faint outer glow of black using layer effects to give your object depth. And there you have it - a grungy interface!