Are you guys sure of what you speak? Is it an insult for Muslims, or for Arabs? In other words, is it a religious or cultural thing (and yes, I understand Islam is as much about culture and politics as it is about religion)?
Does a non-Muslim Arab, or a non-Arab Muslim from say SE Asia feel the same way about the act of striking somebody with the sole of their shoe as a Muslim Arab?
I'm asking only b/c people too often spout what they think they know.
The Ismail Ak theory looks more promising to me.
I have heard about showing the bottom of the shoe being an insult, but I confess I haven’t made a world tour to check it. Here’s a piece in the WSJ that suggests it holds for the Arab world, and apparently also at least as far as India:
http://www.opinionjournal.com/taste/?id=110003324