If you can maintain a competitive advantage while open sourcing your code, then I think it's better to do so. I mean your whole shtick is teaching people to code - so open sourcing follows naturally from that.
Keeping up competitively means keeping your design better than everyone else's and keeping your feature set better than everyone else's. If you do that and open source your code, you'll be alright.
Thanks for that data point bennyg, appreciate it. Our intention is definitely to do as you describe (keeping ahead of the pack in terms of features and design). One thing YC is going to allow us to do is invest heavily in those areas in the coming months so as to differentiate ourselves.
Keeping up competitively means keeping your design better than everyone else's and keeping your feature set better than everyone else's. If you do that and open source your code, you'll be alright.