There is an easy and fast trick/ feature/ bug to make it happen.
—I’m sorry for any false conclusions i made by mistake, please correct me when i’m wrong.
Don’t consider this to be a tutorial, i just found out by luck and want to share with you. Don’t be mad when you think this is the wrong forum section for this post. I didn’t intend to make it so long :o and I’m sorry if this is an old hat.—
So here are two things i want to share because it speeds up my workflow.
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is for kickstart painting, 2. is mirror painting.
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Fast-Unwrapping. (<-It’s “Smart UV Project” with specific settings.)
Pro:
It’s Fast. No thinking about “marking seams”. No overlapping Uv-layout/islands. No or nearly no stretching.
It works great for all kinds of in-progress models/geometry, where you just want to paint on quickly.
Contra:
Not so suited for example for cloth-mapping, but there is a workaround! -
Texture Paint Mirror.(<-That’s the actual news.)
The best thing about it: it is independent from your uv-layout, so you even don’t need “symmetrical uv-islands”, -
Fast-unwrapping:
Just select your model.
Go into “Edit Mode” select all faces by hitting “a”.
Hit “ctrl-e” select “Mark Seam”. (So now every edge should have a seam!)
Now hit “u” and select “Smart UV Project”.
Set “Angle Limit” to “1”. And Set “Island margin” to “0.01”.
Hit “Ok”.
And thats it for the unwrapping part.
Now assign a texture to your model the way you like.
For example with the “Texture Paint Layer Manager” addon by Michael Williamson. It saves time 
You shouldn’t get any visible seams, but if you do: Make sure all Faces are planar and maybe
change in the “texture paint panel” “bleed” to 5.
Now it should be possible to paint onto your model without ugly seams and without wasting too much time “marking” them.
So lets go to the mirror painting part…
- Texture Paint Mirror:
Just add a mirror modifier to your (already symmetrical!) model.(That is the actual trick!)
Go into “Texture Paint Mode”.
And (Important!) turn “Occlude” in the “texture paint panel” off!!
That’s it.
When you now paint on your model the paint will “magically” be mirrored. Even though our fast created uv map is in no way symmetrical (as it would be needed with other tricks to achieve mirror-painting), and we spend just a few seconds on unwrapping.
When you turn off the visibility of the mirror-modifier you can paint as usual.
So you can, at any time switch mirror-painting on and off!!
Note:
This works only with “projection paint” on!!
That’s why for example the “Soften-Brush” doesn’t work with this trick because it doesn’t support “Project Painting”.
Maybe Psy-Fi can do something about it during his GSOC 2012. :o
Maybe this whole thing is just a bug and will not be possible in the future, after code-changes?
Maybe its just some unwanted side-effect and gets removed??
I hope not.
For now it’s just a fast way to quickly mirror paint onto a blank-model, in just a few seconds. And you can think about the “actual unwrapping” later.
3 more things:
1.)My understanding is that with this “fast-created uv map” the space and/or resolution of a texture will be used very well and you get less to no stretching, because the uv-islands have the exact same shape as their corresponding faces. And they are evenly distributed.
2.)(This is also the above mentioned “workaround”.)There are many reasons for a standard “clean-uv-map” with propper marked seams. For example to use it for “cloth mapping” or to process it in Gimp or Inkscape…
So when you don’t like this way of “fast-uv-map”, and you need a proper clean unwrap, for what ever reason…
You can do that at any time and then bake the texture from the “fast-uv-map” to a texture with your “clean-uv-map”.
So with this method you lose nothing, you just have a way to quickly kickstart the painting process.
3.)I noticed one thing related to the fast-unwrapping. As long as you are in texture mode the texture has no visible seams in the 3d-viewport.
As soon as you go into object mode you get very visible seams, practically for every face!
Also this is not a problem when you render with cycles, because there you will get no visible seams.
With Blender-Internal: again you get/see those seams.
I think i just didn’t find the right settings to compensate for that.
Question to Psy-Fi and everyone else:
Would it be possible to change this side effect of “mirror-modifier in combination with project paint”, into an intended feature?
Because there are only two things that keeps this trick from being perfect.
One is, the above mentioned: the “Soften-Brush” don’t work.
And sometimes due to the fact that when you mirror an already-symmetrical model, you get those artifacts when two polygons fight for the same space.
But you have these artifacts only in the 3d-viewport while the mirror modifier is visible. It doesn’t affect the actual painting. The paint result is very clean. So no big problem, just a little bit distracting.
Video:
I made a little video showing the process.
Like said before, it is important to turn “Occlude” off, but it is also good to know that when you additionally turn off “Cull” and “Normal”, you can paint through the object.
You can do things like make the brush very big so that it covers the whole object. this way you can change the color of the object with one stroke/click. That’s how the bunny gets his brown color and the white-head gets multiplied with a “skin-color”.
“Y-mirror-paint” works also, as you can see when i paint on the hat of the bunny.
At the end of the video i messed up, because sometimes the undo-function in texture-paint mode don’t work as intended. After undo a paint-stroke, the image still has artifacts of that undone paint-stroke.
That’s it, and thank you for reading.
