Blizzard has set May 19 as the deadline for any add-on devs to comply with the company's new add-on development policy. Here's Nethaera's post on the US World of Warcraft forums:
On March 20, we released the new add-on development policy to the public as an ongoing effort to help ensure add-on integrity, safety, and quality for the community. Since that time we have been in touch with many UI development communities on what the policy means to them and the continuing development of these add-ons.
We would like to announce further that we are providing a sixty-day grace period beginning at the time of the initial release of the policy for UI developers to comply with the new policy. After May 19th, Blizzard Entertainment will begin to enforce the Add-on Development policy, and developers found to be in violation of this policy will be contacted directly.
We can probably guess what being "contacted directly" entails.
The new add-on policy prohibits add-on authors from charging for their add-ons and from soliciting for any donations in-game. A policy against in-game advertising was also added in.
Since its announcement back in March 20, several popular mod authors have already stated their intent to cease development, while others have switched to models that reflect the new policy's restrictions.