Grooveshark has been in legal issues with major music labels. Like YouTube, Grooveshark depends on users to legally upload music that can be enjoyed by the community. These legal issues has caused them to get banned from the Apple and Google mobile app stores.
With the launch of HTML5, they have found a way to work around not being in the app stores. Users can access the app through their mobile browsers. The app itself is simple in design right now, but functional. But we can expect that to change in the near future.