If you couldn't tell from the OP, I always use an Oxford comma. And, like Shlup, I always notice when other people don't use it and it bugs me. Even regions that don't use the Oxford comma, like the UK, require its use where the lack of the comma can cause ambiguity (such as in the OP's examples), so I don't understand why it's not just always used. Silly Brits.
And I was an editor on a law journal for two years, which like all good American publications required the Oxford comma, so I became used to correcting people's omission of it.
