observe_field using :on => select not working

Is there a known bug with observe_field using the :on parameter?

I want to observe a select list, and only update a child select list
after someone has made a selection, so I want to use the onSelect
event. However no matter what I set as the value of :on it still
behaves as if it is set to change (the default).

I came across this http://joshhuckabee.com/taxonomy/term/5 which
implies there is a bug - I even change my prototype.js to match his
changes, but it still is not behaving as I would have expected.

I have used onSelect before in select lists, and it definitely only
fires when you actually make the selection, so I am very confused!

I was frustrated with this as well; there isn't good documentation
online. Or, if there is, I couldn't easily find it. I got this
information through bugging a bunch of people in IRC.

It might be better to do it this way:

Interesting - I will give that a go tomorrow and hopefully it works
out better. So frustrating with the observe method, as its so simple!

I got the observe_file method working - upon re-reading the docs, it
seemed that not setting the frequency parameter was the way to achieve
what I wanted, and that onchange was infact the correct event to check
for when using a select list.