Accented Memories

I have had a theory for a long time that people, consciously or unconsciously, change their accents depending on who we are talking to and what we are talking about. This is mainly because … I have always done it myself! Turns out I'm not alone - according to the Economist, accent shifts are fairly common.
Using a video example featuring author Bill Bryson (whose work you should get familiar with if you aren't already - he's a fantastically engaging writer), who grew up in Iowa but who has spent most of his adult life in England, these subtle shifts become evident. Bryson's inflections change when discussing something neutral (where he slips into an English cadence) versus something from his childhood (hello Iowa!)
Do any of you guys have a fluctuating accent? Has anyone ever noticed and said you're pulling a Madonna? (And thanks to Sarah for pointing me to this link!)