I put together a collection of different grime/dirt brushes for photoshop. I use these brushes in almost every project I ever do, they are great to paint dirt/scratches etc. If you want to make the edges of your text a little bit worn, no problem, just create a mask and start painting with these brushes in it. If you have Photoshop 7 be sure to try the “shape dynamics” when using these brushes, its great to paint scratches etc with.
I made them in photoshop 7, but hopefully they should load on older versions too.
Download it here http://www.renderart.net/files/dirtbrushes.zip
here is a sample of the brushes: