if you want to take nice photos where the background is blur, you'll want a good "portrait lens". This means a lens with as large aperture an aperture as possible (especially at the tele part of the zoom range). Image quality at the tele part of the zoom range will also be important. If you have money to splurge, some recommendations for Canon are here:
http://www.the-digital-picture.com/Canon-Lenses/Canon-Portrait-Lens.aspx. For cheaper alternatives, go to a shop that would let you try out the lenses until you find out which one you like, read the lens reviews on Amazon.com or sites like
www.the-digital-picture.com, dpreview.com. The kit lens might provide "acceptable" background blur at the far end of the zoom, but you'll have to try it out to decide whether it's "acceptable" enough for you. I don't think Pentax provides as large a range of lenses (i only saw 5 choices of lenses on dpreview.com) as Canon.
In terms of the chip, i've used Digic IV (on a 500D) and i find it's really really good (not sure how the Prime II with the K-x compares). I get some really good night shots+flash with the Digic IV engine.
In terms of image stabilization (IS), Canon IS lenses provide about 3-4 stops of image stabilization which I think would be better than the K-x which provides stabilization on the body. Of course lenses with image stabilization are usually considerably more expensive (but the difference is probably quite small for Canon kit-lens).
Get the K-x only if budget constraints are important for you. Otherwise get the 550D. There's a reason why people choose Canon.