I'm surprised that Longmore & Worrell's paper hasn't come up in this discussion:
Longmore & Worrell, 2007, Do we need to challenge thoughts in CBT?, Clinical Psychology Review, 27, 173-187
available at
http://www.actmindfully.com.au/upima...hts_in_CBT.pdf
(Although, I thought Behavioural Experiments are specifically designed to test thoughts & beliefs, as they don't generally fulfill criteria for exposure.)