As HN: Open source mobile apps, charge for Saas backend?

We provide a SaaS service for expense, receipt, time, and mileage tracking for businesses ( http://xpenser.com/ ). The revenue model is simple - we charge per user, per month, for the backend service. We also offer mobile access from just about anything (SMS, email, IM, twitter, voice, etc). We've been discussing making our mobile apps open source. We have a lovely userbase that includes some very talented technical folks that are already using our APIs to build some great stuff (including mobile apps), so we figure by opening up the code base we'll give them even more to play with. The downside is that people could copy our code and build competing apps. We're not too worried about this since we make money on the backend system (not the apps), but then again we don't necessarily want to spawn lots of similar looking apps to crowd the marketplace. Thoughts or advice on this? Do you know of others successfully doing something like this? Is there a reasonable license that allows us to make the source open but prevent people from using it for competing services?