If you mean Flip Horizontal/Flip Vertical we mostly already have it but the user
interface for it is horrible.  

take a simple shape, like the ones in the Assorted sheet
(and if you have no menubar turned on, you now need to)
*middle click* on the object and the context menu will appear
(this is different from the menu you get when you right click)
if the object supports flipping you should see Flip Horizontal and Flip Vertical

*** This bug has been marked as a duplicate of 60331 ***

yes, of course, i meant: flipping not mirroring.

What is "the Assorted sheet"? 
I can't find any shape which supports flipping.

My menubar is on and  
middle click does nothing but moving the view.
Left click shows the same context menu when menubar is off and middle click is
used, i think.

BTW: You should add a comment in the documentation about this.

If the menubar is on then you only need to right click to get at Flip H/V

The Jigsaw shapes can be flipped, so can most of the Network shapes 
(none of the UML Shapes or ER shapes or other "programmed objects" can be
flipped though)

I am not in charge of Dia by the way, in case you thought I had the power to
solve this problem.  I just try to help out.  

Normally I would just document the behaviour but I really do not like it at all
and I dont want to document behaviour this badly designed.  At best I might
include it in the FAQ because it does get asked a lot.  
In bug 60331 I propose putting Flip in the Object menu (amongst other things)
which should make the feature easier to find.  
I want to change how this works and fix it properly but changing it is difficult
when more difficult.  

If you are a programmer and are intersted in helping out please do as the Dia
developers are always grateful for patches.   

I really don't like that behaviour either, but it's hard to add full
rotation/flipping (they would probably go together).

Lars my point has been that it would be worth putting Flip Horizontal and Flip
Vertical in the Object menu to make users more aware of this functionality even
though only the shapes support it and none of the programmed objects.  It would
still be less than ideal but it would not be any worse even though it is only a
small improvement.  

Full rotation and flipping is a whole other story.