Owning several Pentax (K20D, K-5, K-3) and a Sony (SLT A55V), I can add a little to Itai's:
Pentax has a unique gimmick: An add-on GPS device (well, my Sony has GPS already built in), the O-GPS1, that is able to track stars. So, if you like to experiment with shooting stars at night, this is quite a great accessory. You can find example images on Flickr (tag: Astrotracer) and on Pentaxforums.com.
All Pentax DSLRs have two dials - one in front, one at the rear, and you can configure them, individually for each exposure mode (P, Av, Tv, X, M etc.) to control either time, aperture, exposure shift or ISO values directly. Very convenient - e.g, in P mode, it usually selects a middle value for T and A, but if you don't like it in a particular situation, you simply turn one of the dials to immediately change A or T to a chosen value. No need to switch from P to A or T for that.
There are a lot of customization options. E.g, If you are using a flash (internal or external), you can choose whether you want to take a shot if the flash is not ready yet at the time you press the shutter button - if you allow it, it'll take the shot as if the flash was off, with adequate exposure. For P mode, you can preset the preference for the choices of ISO, T and A: Speed (short T), small or large aperture, MTF (optimal A for lens). Can't say if the other brands offer so many choices, though my (cheap) Sony at least doesn't.
The new top-of-the line model, the Pentax K-3, now enables remote control (including Live View) abilities with the "Flu Card" as an accessory. Though, as of now, its flexibility is far from what the pro models from Canikon offer.
Like Sony, the Pentax K-3 now also has focus peeking in Live View. Older models don't, though.
As for the entry-level Sony models, I can say about mine that it is significantly lighter than any Pentax DSLR. There are also light zoom lenses from Sony. This is of advantage if you tend to travel with your camera in your bag or backpack.
The swiveling rear monitor on the Sony SLTs can be useful as well. Pentax does not offer this at all so far. OTOH, I can connect a portable LCD monitor to my Pentax via its HDMI output if I need the flexility, e.g. for macro shooting.
The Sonys, not being true SLRs (because they do not use a fold-up mirror) can, due to their design, focus faster while recoding video or taking shots in rapid succession ("drive mode") because they can use phase-detection sensors while the sensor is recording. Pentax can't do this quite as well.