It's a start

http://richarddawkins.net/articles/4975

Apparently a church has been found to be spending too much time politicizing rather than proselytizing. Political animals have to pay taxes.

Next up - the Mormon Church:
http://andrewsullivan.theatlantic.com/the_daily_dish/2010/01/the-prop-8-trial-document-dump.html

and

http://andrewsullivan.theatlantic.com/the_daily_dish/2010/01/the-mormon-hierarchys-plausible-deniability-ctd.html

Pump millions into a political activity and you're a political group, not a church eligible for tax-exempt status.