In the Project Server 2010 solution it is now possible to work with your tasks/project plan in edit mode directly from your PWA site. Not all functions are available and therefor it should only be used if your projects have some rather simple requirements in regards to the controlling of the progress. Microsoft have released a document that explains the difference between the PWA and the Project Pro client when it comes to the planning of a project.
Document: PRO_PWA_comparison