In the wake of the September 11th attacks, Robert Pape, Professor of Political Science at the University of Chicago, spent two years establishing a database containing details of every known suicide bombing or attack - 315 in all - since 1980. (His findings were summarized in a paper for the American Political Science Review in August 2003, and published in his book, Dying to Win: The Strategic Logic of Suicide Terrorism, in May 2005. The importance of Pape's database was that it was a comprehensive record of suicide attacks worldwide. Usually, the only suicide attacks that we in the western countries care about are the ones that target us, and that allows us to make the association between Islam and suicide bombings, because it is generally Muslim countries that we are in conflict with right now. But by documenting the phenomenon from a global perspective, and taking into account the suicide attacks in far-flung countries that don't usually make it onto our radar, Pape concluded:

The data show that there is little connection between suicide terrorism and Islamic fundamentalism, or any religion for that matter. In fact, the leading instigator of suicide attacks is the Tamil Tigers in Sri Lanka, a Marxist-Leninist group whose members are from Hindu families but who are adamantly opposed to religion...

-- Dying to Kill Us; NYT, 22 Sept 2003.

Pape found that there was an indirect connection between religion and suicide bombings, inasmuch as suicide attacks often occurred in conflicts in which an occupying power of one religion was in control of a land where the occupied population was of a different religion. But once again, Islamic fundamentalism did not account for that connection, because the connection existed regardless of whether or not one of the warring parties was Muslim. For example, in the period Pape studied, suicide attacks occurred in conflicts pitting Hindu versus Buddhist, Muslim v. Christian, Sikh v. Hindu, Muslim/Christian v. Jewish, as well as in conflicts where there was no religious divide and both protagonists were avowedly secular (as in the campaign by the Marxist Kurdish Workers Party against the secular state of Turkey).

Pape's conclusion was that the most common motivation behind organizations that used suicide attacks was not religious fundamentalism, but nationalism:

There is not the close connection between suicide terrorism and Islamic fundamentalism that many people think. Rather, what nearly all suicide terrorist campaigns have in common is a specific secular and strategic goal: to compel democracies to withdraw military forces from territory that the terrorists consider to be their homeland.

Religion is rarely the root cause, although it is often used as a tool by terrorist organisations in recruiting and in other efforts in service of the broader strategic objective. Most often, it is a response to foreign occupation.

-- What we still don't understand about Hizbollah; The Observer, 6 August 2006.

So while it's true that we don't hear about suicide bombers of different religions (or of no religion at all), it's not because they don't exist.  Perhaps they are underreported over here because our news media thinks we are not interested in terrorism that targets people "not like us", or maybe there's a more ideological motive, i.e. the desire not to question the "terrorist = Muslim" stereotype.