While there is not yet a Project Server Online offer, there is a way to use the power of SharePoint Online to work with Project Professional (client side).
As part of this integration, there is task list synchronization, document sharing (of course) and of course user interaction through Lync client (if used).
For more details go to http://blogs.msdn.com/b/project/archive/2011/06/29/microsoft-project-professional-2010-and-sharepoint-online-in-office-365.aspx