I found this old article
http://news.bbc.co.uk/1/hi/health/2300395.stm
Fascinating isn't it? I hadn't heard of it before either, and don't know of anyone who has suffered with it..most odd!
I lived in Manchester for a few years and began to take on the accent, although not severely. My southern relatives noticed a change, but to the Mancunians I still sounded like a 'southern git' ;o)
Whenever my dad goes to see family in Wales he soon changes his accent and speaks like a native ~ even the Welsh can't tell he is English.