All Twitter APIs will soon use HTTP error code 400 for calls beyond your rate limit. One of Twitter's most popular services, the Search API, currently uses error code 503. You'll want to update any of your applications that use Twitter search so that they're looking for the right code.
With well more than half of the Web's traffic now starting with a mobile device of some sort, Google is looking to keep the pressure on when it comes to improving the performance of mobile Web apps. So, now it's starting a certification program so developers can prove their proficiency to customers.