Quote from Mark Shuttleworth

In the panel, you’ll see that the top left corner is now consistently
used to close whatever has the focus. Maximising a window keeps the
window controls in the same position relative to the window – the top
left corner. We have time to refine the behaviour of this based on
user testing of the betas; for example, in one view, one should be
able to close successive windows from that top left corner even if
they are not maximised.

It’s taken several releases of careful iteration to get to this point.
Even though we had a good idea where we were headed, each step needed
to be taken one release at a time. Perhaps this might make a little
clearer the reasons for the move of window controls to the left – it
was the only place where we could ultimately keep them consistent all
the way up to a maximised window with the title bar integrated into
the panel. I’m confident this part will all be settled by 12.04.

As part of this two-step shuffle, the Dash invocation is now
integrated in the Launcher. While this is slightly less of a
Fitts-fantastic location, we consider it appropriate for a number of
reasons. First, it preserves the top left corner for closing windows.
Second, the Dash is best invoked with the Super key (sometimes
erroneously and anachronistically referred to as the “Windows” key,
for some reason ;-) ). And finally, observations during user testing
showed people as more inclined to try clicking on items in the
Launcher than on the top left icon in the panel, unless that icon was
something explicit like a close button for the window. Evidence based
design rules.

Paraphrasing Mark
User testing showed that the original location confused people ,so the dash in 11.10 was an attempt to resolve this.

For 12.04 - the dash button remained within the Launcher.