This cool feature was first told to be included in 10.10. According to this answer, they were postponed to 11.04, but it turned out they have not appeared there. Now we are after feature freeze for 11.10, and still there is no sign of them. So what's up with them?
I haven't heard a word about them for a long time. Is this idea abandoned? Note it was initially the reason for moving the window-ops buttons to the left corner. Or maybe they are under heavy development and soon will be included in 12.04?