The full Instapaper API is now available for developers. "All previous API functionality will remain free and will work for any account. These methods can only add pages to people's Instapaper accounts, as implemented in many feed readers and Twitter clients. This part of the API is not changing, and its now called the "Simple API"."
The full API also allows developers to let their app' users read data from their Instapaper accounts. The use of the full API will also be free to developers, "all developers can use the Full API, but it will only work for customers with Instapaper's $1/month subscription memberships."
More information about the full Instapaper API is available in the documentation.
Instapaper's own paid and ad-supported free iPhone and iPad apps in the App Store will continue to work for all customers.
The very first app to take advantage of Instapaper's full API is Stacks for Instapaper, a Windows Phone 7 client. You can get it for $2.99 if you search for it in the Windows Phone 7 Marketplace, or usign this link, or by scaning the Microsoft Tag below:
[tags]instapaper,instapaper api,stacks for instapaper,winp7,wp7[/tags]