We've had a number of reports of people appearing to log in as someone else, this is a simple (**** edit ok not to simple as it turned out ****) caching issue which we're fixing now. There is no way to access others pm's (they are not cached for obvious reasons). Your pm info isn't at risk, it simply doesn't run that way. If you try to pm or post when the username is altered it will revert to your logged in account when the post / pm is made and have your details on that post / pm. Closing other threads and pointing them all here.