QI, which is short for Quite Interesting, is a British comedy panel game TV show hosted by Stephen Fry and Alan Davies.
It's been running for some years now, and I've just found out this fragment in which Fry, David Mitchell, Rob Brydon, Sandy Toksvig and Alan Davies discuss how we perceive accents. Amongst the accents discussed, it seems that unlike Geordies, Brummies are not considered to be trustworthy.
For those of you that don't know (I didn't), Geordie is a regional nickname for a person from Tyneside region of the north east of England, or the name of the English-language dialect spoken by its inhabitants, whereas Brummie (sometimes Brummy) is a colloquial term for the inhabitants, accent and dialect of Birmingham, England, as well as being a general adjective used to denote a connection with the city, locally called Brum.
Verbum Sapienti led me to the discussion of this subject in Jeremy Vine radio show on BBC Radio 2, December 8th 2010. In it, Jeremy Vine discusses the distinctive Birmingham accent and considers whether Brummies are considered "thick" simply because of the way they speak.