So, here are the functions:
And here's the bookmarklet:
Just drag the link above to your menu bar, and click on it in any site. The bookmarklet will appear at the top of the page. It takes a pixel amount, and any css selector you can think of, so it will return the conversion relative to an element. For example, on this site, if you use "body", 16 pixels = 1.14em, but if you use #content p, 16 pixels = 1.23em.
It's probably not perfect, so please let me know if things aren't working correctly. Although, I can tell you right now it's not going to work on ie6, so don't bother trying it there.
The bookmarklet makes use of Dean Edward's cssQuery, a slick little script to retrieve elements based on CSS selectors.
All in all, this was a fun experiment, and I hope you find it useful. Have fun.