Inequality is the wrong diagnosis. As msandford pointed out, it's more likely to be a minimum standard of living issue and having enough to lose. If inequality was a main factor, you would expect Pakistan to have similar crime rates as Sweden, when in reality they're not even close. (PG has an essay on inequality being a boon to a nation's economy: http://www.paulgraham.com/inequality.html) You could also look at the religiosity of a country, but the trend there is that lower standards of living correlate with higher religiosity. (http://www.gallup.com/poll/142727/religiosity-highest-world-...) I'm more inclined to think the causation goes from low standard of living --> religiosity rather than the other way around, given the United States. However there could be other deeper factors that give rise to a low standard of living, such as a nation's average IQ -- you're not going to see a nation with average IQ of 70 outperform let alone match a nation with average IQ of 100 in terms of standard of living.
Good point regarding inequality vs absolute standard of living, but you're also talking about inequality within national boundaries. A good bit of the resentment of the Muslim world towards the West stems from perceived injustice and power imbalance on a regional or global scale.
There is also a big difference between inequality as a ratio of rich to poor, and actual conditions. The poor in Sweden are better off relative to the rich than the poor in Pakistan.
"For every complex problem there is an answer that is clear, simple, and wrong."
Inequality is definitely a contributing factor, perhaps even _the_ contributing factor, but don't count out religion.