New 'AdWords API Beta Program' has been implemented from November 1, 2006. Rohit Dhawan, Product Manager at
Google, announced "..the current developer quota limits have been abandoned and developers will begin to accrue charges at the rate of $0.25/1000 quota units consumed (or local currency equivalent). We believe these changes will result in more flexible and scalable system for quota allocation and consumption."
Google Adwords API Support explains all about API.