Pronouns and Case - But Me, Or But I.

But is usually a conjunction: I like the idea but I think it’s impractical. But when it means ‘except’, is it still a conjunction?
Nobody but I took the idea seriously.
Or is it a preposition, which takes the object pronoun me instead?
Nobody but me took the idea seriously.
Unlike the as and than examples, these cannot be expanded into a reasonable sentence by adding an extra verb: X Nobody took the idea seriously, but I did. So but seems to be a preposition, and but me to be logically correct. If you remain uneasy, you can always avoid the whole issue by rewording:
I was only person who took the idea seriously.
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