Right you had a question. The short answer is that 'No,' there is no way to copyright your app idea. Copyright doesn't protect ideas (despite some incredible misinformation on the web). For example, if you have an idea for an app that identifies poisonous mushrooms or makes gastrointestinal sound effects, anybody can use a similar idea as long as they don't copy the way you expressed your app. That's why success in the App Store is often based on getting the best version of something out quickly, and then popularizing it so that it turns up highest in App Store search results.
Paranoia strikes deep. If you're paranoid about the theft of an idea, you can ask the company to sign an NDA. For that to work, you need to be able to prove that the idea is a secret -- that is, it's not something commonly known in the App trade -- and that you've been treating it with secrecy. You can get free sample NDAs and explanations at our special website we set up for that purpose.
Hiring someone. When you hire a company, you should also make sure that they assign all copyright to you. If you're concerned about ripoffs, or just want to be extra-prudent, file a copyright application once the app is completed. We should also mention that some ideas (provided that they're not abstract) can be patented, which would likely be a waste of money and effort since it would take almost two years to get the patent and the shelf-life of your app would likely be over. Finally, although we can't guarantee it, in most cases a legitimate app developer is unlikely to steal an idea because it would be very bad PR for the developer's business.