JSONP is a very neat way to get JSON data from other domains. The concept was brought by Bob Ipolito in 2005, and today it is widely available from API providers.
The big issue about JSONP is security. If you inject an unknown script in your page, you give to the script author a potential way to read the entire page content, scripts, cookies and data.
So, if you think, like us, the cool way to build web apps today is about building services and aggregating them with external APIs in Mashups, you have a problem. How can we securely fetch these nice services available everywhere?