Serious question, if I outsource my app idea to be made elsewhere through a freelanceer or agency, should I be worried? How would I protect myself here? NDA?
Trust is important. Go with your instinct (this is why a face to face meeting, if only initially, is so important - you cannot evaluate someone virtually, the nuances get airbrushed over). Basically, if we were to screw a client, the grapevine would know. Relationships are everything and that is why we get work that others could do 80 to 90 percent of.
We do sign NDAs from time to time. But really most of those NDAs are to protect the client from us revealing we did the work.
Truly unique ideas, you will need to build your own trusted network. And sell the developers on the idea (they will not commit just for money, you want to capture their heart).