This Reuters article, Switching languages can also switch personality, is a pretty interesting idea.  However, while I haven't read the original study, the article's lay explanation seems to have an obvious flaw.  It sounds like they studied native Spanish-speakers and discovered that, when speaking in their native language, they were more assertive, self-sufficient, and extroverted.  Is that really a sign of a personality change?  It sounds like a consequence of the mental challenge of speaking a foreign language.  I'm certainly less assertive when I speak German; I'm just lucky to get an idea across.  It would be interesting to see scientific proof that switching accents can switch personality, though.