The upgrade path has a number of factors to it. Pentax doesn't have a full frame camera, it has both the APS-C sized sensors and a medium format camera (that's just awesome, would love to have the money to buy one). It has two different levels of APS-C cameras.
I got the chance to play with a full frame Canon that belongs to a friend of mine. I was reviewing some shots he took, along with looking through the viewfinder and seeing how it was to handle. It got very heavy, very quickly - there's no way I could imagine taking it out for a day-hike or carrying it all day while being a tourist somewhere (my two favorite activities). Plus, with the grip I found my short fingers couldn't handle the heavy weight and operate the zoom, too. Granted, I'm a small female and these are objections a big, strong guy wouldn't have but they are a very significant disadvantage for me.
So that leaves me with the APS-C sized cameras. Pentax does have an upgrade path - they've always kept two levels of cameras - an entry level camera like the Kx and then a "higher", more feature-filled camera (currently the K5). Their higher camera has a great deal of capability - far more than what I'll ever be able to figure out, and it's been highly rated by professional reviewers. So in my opinion, having 5 or 6 models, the top ones being ones that not only can I not afford but also can't use because of the ergonomics doesn't seem to be a reason for buying that brand. Having an excellent camera with more features than I can figure out how to use along with all sorts that are very useful, and one that I can afford to buy means more to me. So I think, at least for me, the argument that Pentax doesn't offer as much upgrade path (i.e., no full frame camera) is over-rated. And if I want to dream, there's always the 645D to dream about.
If you think that you'll really need a full frame camera, can afford it and the ergonomics will work for you, then skip the Pentax. But since for my purposes I can see no advantage to full frame (there are others who need it by the way, I'm not saying that an APS-C camera can equal a full frame for certain things), it's ergonomics are all wrong for me and I couldn't afford one anyway, I'll stick with my Pentax cameras.