How easy is the mod, and what lenses will it work with?
The mod was pretty easy. I saw this article and video about this guy from the middle east who experiments with different lenses and he used the MC Zenitar M2s, so I used that one.
You just flip the center element over and reassemble everything back together as normal.
I don't know if it what results you can get with other lenses, but it may be interesting to try. I used a special spanner to disassemble the lens, though. It's much easier to use than a knife blade!
edit/ Check out the Flickr group "Lens Hackers". Lots of cool stuff in there. One guy has some cool results with his Helios by inverting both the front and rear elements.