I have no idea to what extent they actually get used for money laundering, but churches are still the only form of nonprofit in the US that automatically gets exempted from having to file a tax return. I'm sure the whole season five plot of The Wire where this was how drug lords were funneling money to the local politicians was not entirely fictional, given how everything else in the plot was based on the writers' real life experiences.

Of course, even on The Wire, this was small time compared to the money laundered through urban reclamation and seafront development projects.