Regardless! The K3 uses its "ultrasonic" sensor cleaning system, which really does nothing more than vibrate the sensor very quickly, to shake and thus blur the image slightly. This gives it the effect of a low-pass filter.
An increasing number of cameras recently have been ditching the low-pass filter for two reasons. It absorbs a small amount of light, meaning that cameras without the filters do slightly better in high-ISO situations. It also adds a small degree of sharpness. There are many photogs who see this as "critical" sharpness, or that last bit of a good photo that gives the image an edge or "bite."
Generally, I would make fun of such ridiculous and idiosyncratic language, but it does actually make sense. That last little bit of sharpness in an image, when multiplied over an entire photo, can actually make a noticeable difference, even to an untrained eye.
So, how does the new K3, which gives you the on-the-fly option, hold up? Imaging Resource has posted its images, and we can actually draw a few conclusions.
First, the JPEG ISO performance is exactly where every other competitive camera is in the segment, making me further think that, at least currently, the ISO wars are over. In case you missed it, Fuji won. The X-Series of cameras is so far ahead of the pack as to really be... huh?
I captured Imaging Resource's images and did a huge blow-up of their corner resolution, where the effects are going to be smallest. For me, this is the most interesting part.
It's impressive that even here, there is a difference. There is a small difference, to be sure, but one of the most eye-catching elements of the comparison is the slight increase in edge contrast along the black bars, as opposed to merely an increase in resolution. Right there, right in those black bars, is the "pop" that people talk about in images taken without the AA filter. That increased overall contrast is noticeable. As is easily visible even in the low-res preview above, the corners, but not in the center of the image, also suffer an increase in chromatic aberrations.
That is a very interesting aspect to an AA filter that I had never thought of. It acts as a small band-aid over slight variations in the alignment of wavelengths of light within the lens, thus hiding what chromatic aberrations would be there. It hides small defects in the lens design.
I recommend going over and viewing the full series of images at IR. There is a similar difference in the center of the image, where resolution is highest, and well worth a look.