For those who put a lot of hard work into testing/developing-against the API, I thought it would be nice to reward them with a badge.
The more I thought about it, I began to think:
Why don't we repurpose an existing badge?
Since there is already a badge for participating in the private beta of SO, why don't we change it to say
This badge is awarded to those who participated in testing something on StackOverflow.
Of course, the wording needs to be changed, but you get the idea.