This is a nice bit of frustration you might run into in BusinessObjects XI R3 WEBI.
Here's the situation : You have a report -- any report. When you move a table, delete a column, delete a cel, the undo button does not light up.
It's not a bug really, it's normal behaviour.
If you accidentally click the Show/Hide Filter toolbar button (to the left of the Drill-button) -- for some reason, the undo-function is de-activated.
So, if you run into this issue -- just de-activate that filterpane button.