I tend to get bored if I stay in the same job too long. Also cash, promotions, and respect are easier to come by when you switch companies. Sad, but true. My progression has been tech support, NOC, Network Engineer, Windows Admin, Network Eng/Sys Admin, Sys Admin/DBA/Network/Developer/IT guy, Application Operations, and finally head Application Engineer which is mostly capacity planning, architecture review, project management, and trying to catch issues before they take the service down. Most of what you learn at one job can be applied at another job. At the very least you'll be that Sys Admin that actually understands routing or the DBA that doesn't blame the network first.
None of the moving around has retarded my career and I'm somewhat sought after these days because I'm a generalist with a 14 years of experience as well as workign at five startups. Moving around like that might not work for everyone, but I recommend it if you're bored to tears with your current bailiwick and actually like the IT field overall.