I will whole-heartedly say that the K100D is not a premium low light camera. This is based primarily on the consumer-orientedness of this camera and Pentax's current lens offerings.
Wwwwhat? What does this have to do with low light shooting?
To begin, the K100D will only focus using the on-camera focus motor. It does not have the extra contact that the K10D has that allows for using an SDM lens. (SDM= Supersonic Drive Motor; Pentax's answer to fast, quiet autofocus with an on-lens motor)
Second, SDM lenses are as of yet, unavailable.
So? Like you said, SDM lenses aren't even available. The Pentax 10-17 mm is still perfectly usable on the K100D, no? I don't see the problem. It'll take many years before SDM lenses are prominent in Pentax's lineup, so it's not at all for the next few years.
Also, it's not like Pentax K100D has no focus motor. The camera should still be able to AF using an SDM lens, just not with the speed and silent operating benefits that an SDM lens offers (since the camera can't read the signal from the lens). You may ask yourself why someone would buy an SDM lens and not be able to take advantage of the benefits that SDM lens offers? Well, it's because with DSLRs, you're going to buy another camera in a few years anyway. The next camera he gets will be able to take advantage of SDM. While he waits, he can just use the focus motor to AF using his lens, which may not even be bad. I have some very quick AF lenses that don't use Nikon's SWM.
So basically, the K100D will be able to use any Pentax lens with the motor inside the camera.
. Third, only the new "Star" lenses have been announced to incorporate SDM. Star lenses are Pentax's highest quality lenses and are therefore exensive.
Same with every other camera brand, Nikon, Canon, or otherwise.
Four, all of Pentax's current lenses use the old system, the motor is in the camera and AF is driven by that. Also, during focussing, the Manual Focussing ring turns (noisey and inherently slows down the function).
Nothing to add here. You don't like lenses that aren't driven by an ultrasonic motor, right? Sorry, but that's the large, large majority of lenses on the market today. I know USM/SSM/SWM/HSM/etc is faster and quieter, but lets not get out of hand. This isn't an inherent problem with Pentax cameras. This is true with regards to ALL lens lineups from ALL companies.
Personally, I only own one Nikon lens with SWM (an AF-S 105 mm f/2.8 macro with VR), and all my other lenses work great.
Another annoyance, the really good Pentax lenses (Limiteds, FA*, or anything else with a good max. Aperture) can be incredibly difficult to locate. Same goes for their best flash gun: the AF 540 FGZ.
Out of all the things you mentioned, this may be the only major issue I can see. However, if you buy stuff online anyway, it's not a problem whatsoever.
For low light photography, the Nikon D40 (not the D40X) is the best low light camera you're going to get. It fits your other requirements of small size, etc. HOWEVER, can the Nikon 12-24 mm AF on a Nikon D40 or D40X's body? Nope.
Can the Tokina 12-24 mm? No.
Can the Sigma 10-20 mm? Yes.
Can the Sigma 30 mm f/1.4? Yes it can.
The Pentax can use Sigma lenses as well, since Sigma makes lenses with a Pentax lens mount. THe Nikon D40 can only use AF-S lenses, and these lenses don't quite fill the lineup.