Mark:
I use a PS plugin called PhotoFrame 2.5 by Extensis. It comes with more than 2000 frames on the CD that you can combine to get multiple effects. For my gallery show of 60 images that I produced earlier this year, I created 2 templates using a black rectangle and a black square sized to fit my images inside a white border sized to fit 13x19" paper. Next, I combined three frame layers from PhotoFrame over the black edges in both templates to achieve the filed down negative carrier effect you mentioned. I then resized, rotated and flipped each of the 3 frame layers, or used fewer layers, to get multiple versions of the templates. Finally, I layered my images over the black so it would appear as the edges of a negative.
I received several positive comments on opening night about how much people liked the effect of filing down the edges of my negative carrier. They were all surprised to learn that every image was printed digitally.
You can see samples of this technique at:
http://www.jwaldenphotos.com/secondary_show.html
Unfortunately, I think Extensis has now combined PhotoFrame into its Imaging Suite package costing $499., but Adorama appears to still have the single product PhotoFrame for $189.95. There are always cheaper options, but PhotoFrame works well for me.
Jim Walden