You're probably using IE 6+ (wild assumption here) which has problems with logging out, in that it doesn't let you. It stores your login information no matter what, because everyone knows Microsoft Knows Best (tm). It's fucked up, but it's a fact of life.
I know this because it's documented as a bug on the b2 journal tool, which has logout issues with IE 6+. I think it's a compatability thing with PHP/MySQL and IE. But I'm not sure... a search on Microsoft.com for the bug yields only vague hints and no actualy admissions to a problem with their code.
/benny/
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My metaphors are metafives...