Tweaking twentyeleven_posted_on to include categories

I’m trying to tweak the twentyeleven_posted_on function to include the category/categories that my posts are included in, but my knowledge of PHP is limited and I’m stuck.

My site is at https://they.overheard.us and I’d like the line produced by the function to read as:

Posted on [date] by [author] in [categories]

Can someone explain to me how I can make this change? I attempted to modify the function but I didn’t get very far at all.