Wow, I've worked at quite a few places now that I think about it. In the last 15 years, I've worked for 7 different companies, and twice in those years I have attempted my own side business in IT as well. I think changing jobs these days does increase your value as an IT professional (euphemism for programmer in my case). As a number of people mentioned it widens your range of experience, which helps round you out in your approach to software design, programming, languages, tools and most of all trouble shooting and debugging. I think the debugging area is my most affected skill regarding changing jobs. The various and sundry issues I run into on production calls and life cycle testing have really given me an edge when it comes to solving problems quickly.

I've liked all my previous jobs, but if I tend to stay in one place, I tend to stagnate. Usually this is because the project work dries up for my particular group, or the work just becomes routine and the challenges fewer and far between. So, in general, for me, changing jobs every 3 or so years has been beneficial from a career standpoint.