to be honest, I don't know if we even HAVE a datasource that will be returning ALL tasks for starters. Could turn out to be very performance-expensive. IF we had that, then it should be possible to return the data to a user with the corrrect access grants and permissions. Else you might have to create a data source that returns all tasks but be warned...(see above).
there are ways to create data sources to report on task information for other users and not to have impact on the performance - it is just a matter of SQL queries tuning and some database tuning. We have already created such portlets for our customers.
--remember to kudos people who helped solve your problem