Twitter has announced (http://twitter.com/#!/twitter/status/72775840978571264) a new feature today. Users will get an email notifiation if someone they follow retweets them or favourites their tweet. Up until now Twitter has only sent out notifications when users gain a new follower or receive a direct message. The layout of this email changed last week as well. This feature can be turned off however, to save inbox space of popular tweeters.
A lot of Twitter's third party clients already offered this service to help users keep track of their direct messages, retweets, favourites and more. This feature is what third party clients used for a step up against regular Twitter. Twitter's already been hating on third parties that use their API for regular Twitter clients and this seems to be a move to thwart the competition.