Twitter is ramping up its crusade against integration of its services for purposes not solely relying on displaying tweets, namely such ideas as using your Twitter account to find friends on other platforms.
The first service to have its access revoked was Instagram, although one could have argued that this decision was made to retaliate for Instagram's acquisition by Facebook. It's not that easy to put the revokation of Tumblr's API access for finding friends into perspective other than Twitter not being interested in having other companies drawing value from their service. Twitter is only interested in having users and services adding content to their platform.
However, I think that in combination with the recent success of App.net there'll be a significant backlash from developers. Screwing the guys who build the tech to access and/or improve your content is not the best idea. They might simply focus on another service and that's probably what's going to happen as there's already considerable traction amongst tech early adopters to the pay-for-social-network model that App.net wants to deliver.
The trend towards driving off developers is going on however, since especially the case of Tumblr strikes one as particularly unthankful of Twitter given that Tumblr was amongst the major partners to implement Twitter cards. Instapaper creator Marco Arment announced a probable removal of all Twitter features from Instapaper in case his API access is revoked (The usage of iOS's systemwide Twitter integration is still in consideration).
If this will lead towards an ecosystem where the first party Twitter app and the website are the only way to get information from Twitter will be seen. As much as this prediction sounds like a dystopian future, it only takes one look at the 1.1 changes for the Twitter API to understand Twitter wants traditional third party clients to go away; the sooner the better. They even said so themselves, back in 2011.