auto balance flash/ambient

Ernest, I don't believe there is a "feature" to do as you describe.... but my favorite setting for that came through experimentation. A good starting point would be to:

• set the pop-up flash at -2.0 in the menu
• set the ISO at 800
• set the shutter to 1/60
• set the aperture to f/4

From there you should have a good idea what you can tweak next depending on your situation. It's possible to blend things rather nicely after you fine tune it to your situation.

Perhaps Pentax flashes have a bit of catching up to do ? but should be not that far behind flash leaders.

Nikon latest technology provides flash iTTL/ Balanced mode for compatible DSLR (and F6).
This technology provides total automation, so Auto ISO, and varying shutter and/or apertures can be used, depending on camera shooting mode, without operator intervention, if you wish.

When it appeared on the market for the first time, some prominent reviewer heralded the iTTL/Balanced flash technology as "circumnavigating the inverse square law".
http://en.wikipedia.org/wiki/Inverse-square_law

With the introduction of the Nikon SB900 flash and the capability to provide "lighting patterns" from a single point source of light, some again praised the new features as an extra ordinary, and perhaps science defying again.

Best, as always, the truth is in the pudding, so one needs to try and experiment, but certainly the technology is moving on...

Ernest,
What you are describing is basic PTTL--that is the camera controlling the flash process by using the evaluative/matrix metering to make some judgment about aperture and strength of light output.
From my limited understanding, Nikon ITTL does pretty much the same thing, with an added option for spot-metering-based judgment. "3-D metering" is another name for matrix metering I believe. Canon ETTL 2 allows either an evaluative mode or an averaging mode.
Generally I find the Pentax PTTL to be behind ETTL, but it's most noticeable in quick social shooting situations where the rate of keepers has been higher for me with the latter. It's also nice being able to synch at 1/250.
For more serious shooting, it should not be a show stopper for someone who understands flash photography and the limits of a camera's judgment system. Either using the flash's thyristor (A), or shooting in M mode should not be very difficult.
So, based on your question, I think you really need to practice using your Pentax flash in a variety of conditions and settings so you can master the many configurations and make good decisions.

ME

Ok, so, if I understand well, if I let the camera on mode P (=this provides good exposure for background), and if I let the flash on P-TTL, with no flash comp on camera, nor on flash, it should be okay ?
(Assuming that I'm not in special cases, like very low light for background : slow sync , or very high light for background : FP high speed sync)
So you say that in normal situations, P-TTL should provide good exposure and balance for background + subject ? If I understand well, what I described in the first post is just "normal P-TTL" ?

I agree with Michael, A or M mode works quite well, just look at my sample of dog "Nico" in this weeks potw with a simple AF280T flash in auto mode here: http://photo.net/pentax-camera-forum/00ZsIF.

ernest. are you talking about the integrated flash or an additional external one?

I'm talking about using an external P-TTL flash, not in AUTO mode (some flashes have an AUTO non TTL mode, with its own light metering), but in P-TTL mode.

it should be okay ?

Using flash adds a set of additional variables to the shoot. Using an external flash on a DSLR generally is not a point and shoot operation. A lot depends on where you aim or bounce the head, on the unit's power vs. the scope of target you are aiming at, the type of ambient lighting, and your battery's degree of charge. Not to mention skills.

So if you expect a P&S situation that requires minimal judgment from the photographer, then your frustration level will continue. There's a lot of opinion in all of this so YMMV. I never use P on the camera with or without flash because I prefer controlling the camera more and believe I get better results that way.

With a top-notch Metz flash on my K20D in PTTL mode I would shoot in AV mode, always have flash comp +1 in camera, and use a reflector and diffuser as needed. That worked well enough, but later using a much older, cheaper Sunpak 383 on Auto, resulted in equally acceptable results too. But I think I know where to aim the flash head.

ME

I accept the fact it's not as simple as point and shoot.
But how did people do with film cameras, when shoot, read the LCD, reshoot, read the LCD was impossible ?
So there surely exists a method that should work most of the time !
I accept the fact of going to various Manual options.
But I don't accept the need to shoot, read the LCD, reshoot, reread the LCD, etc.
It wasn't possible with film camera. But it was possible to do great photographs with film camera, so I know that a solution exists. :)
I am just shocked by the fact that my old flash with AUTO Non TTL mode works better (more reliably) than my P-TTL flash. This is not normal.