I was going to originally post this as a followup comment to a post from Aaron at Teacher in Development but it got so longwinded I thought I would post it here and trackback to his blog.
Here in South Australia, our education system is run by a curriculum framework SACSA, that is outcomes based so prescribed textbooks are non-existent and in the primary sector especially, teachers are responsible for creating their own curriculum. The framework is the guideline and so, in theory, we have the kind of opportunity you're talking about as a constant. Where your post struck a chord is when you talked about bringing in content. I have a personal example for you. My primary school has a focus on German as a second language (interesting choice for a school of 40% third and second generation Greek background kids) and we have about half the staff skilled in the German language. If you can't teach German to your own class, then you are paired up with someone who can. My class German teacher is a regular junior primary classroom teacher who was worried about the motivation level of eleven and twelve year olds who had already said that German was their least favourite subject at school. So our solution - turn the students into teachers by setting them the task of producing an interactive German computer game that could teach basic German words (colours, numbers, body parts) to our buddy class of five and six year olds. Suddenly, the learning of the second language had a real purpose, the games were for someone other than their teachers. So, for my fellow teacher the decision about what vocabulary to teach was dictated by the class as they created the games in Powerpoint and FrontPage. The kids got a lot more out of that task in terms of German language development compared to a set curriculum from a textbook. And it was a lot more fun!