Method naming and violating single responsibility
It looks like you're building a small DSL, in which event message "Hello" is analogous to, say, Rails' belongs_to :user. The difference is that while belongs_to is only, in a sense, a setter (it can't be called without an argument, and doesn't return any useful value), you've built message as a getter and a setter.

I think this is about context. In your example:

Queue.new :test_queue do
  message "This is a test"
end


..the use of message feels "right." But in this example, it doesn't:

q = Queue.new :test_queue

q.message "This is a test"


..at least not as much. q.message = "..." makes more sense here.

You could just leave it as-is, i.e. allow assignment both ways, using whichever you prefer and letting others use whichever they prefer. The other option would be to make it work one way inside the constructor block and another way on an instance object. Here's an article that's relevant to this approach, with an apropos shout-out to Factory Girl, which itself contains a great DSL.

Hope that helps!