I found an easy and nice Photoshop trick, so easy that anyone can do it with a bit of patience, it was hard to resist and not adapt it for the GIMP.
I made a detailed description of the process with screenshots for every step.
Here is a teaser:
See my page for the full presentation.
As the week-end is coming, see this as a challenge to produce your own works, put on the blogs and amaze people with your GIMP wizardry :p
And maybe post in the comments some links to your work.
Many thanks to my friends for motivating me to write this.