Interesting behaviour for defined?

I've been working with Pete Yandell's Not-a-mock plugin, which I like a lot, and this morning a failing spec led me to some interesting behaviour for the Ruby defined? operator. Try these two things:

defined? arbitrary_attribute

defined? arbitrary_attribute=

With Ruby 1.8.5 on my Mac, the second one fails! From experimenting, it doesn't seem like you can use any setter. The workaround (which I haven't confirmed with Pete yet) is to use 'self.methods.include?("arbitrary_attribute-").