I very much like the Timeline feature of 2010, but find that after a while, it’s not possible to add more tasks to the timeline. If the contents (resource sheet + tasks) of the .mpp file that no longer allows adding tasks to the timeline is copied into a new file, that new file DOES allow adding to the timeline. After a while, it stops working in the new file and I have to start over again with copying and pasting into an empty file again.