I've always thought this (but then, when the infamous "IT DOESN'T MATTER" article came out in HBR, the rest of the business world seems to have started to pay attention) : if you aren't adding significant value to the company that they can't get from anyone else who is younger/cheaper/in-another-country -- your job is not secure.

My father used to work for one of the big defense contractors; Sr. Scientists (his actual title; now more analogous to Engineers) there typically had one or two projects they were working on at any one time. My father typically had at least half a dozen. Towards the end of his career, he'd gotten to the point where they just sat him in an office and people would came to him with problems -- and he'd solve them -- because he knew so much about everything that was going on. Needless to say, when the entire industry was going through massive layoffs in the 90s, he didn't even blink.

When I was in college I got a part-time job in an industry, which lead to a full-time job post college, where I actually learned the business -- and eventually segued to the tech side of things, where I'm senior management in a debt-free company and head of the department. I don't just oversee stuff -- I actually extend and grow the Company using technology. I can do this because I understand not only the tech, but the business of the company as well. And I'm not quite thirty years old.

When talking to people that want to get into "the tech field" (what doest that mean, exactly anyway? Tech support? Programming? Systems design? Networking? Know where you want to go, and what you're getting into!), I always tell people to strive to understand more than just their little corner of the world.

If you yourself are fully modular, you're easily replacable. Become part of the structure and design, not just a cog in the machine.

This is easier in a smaller company, obviously. It used to be "safer" to work at a huge behemoth of a business, because you had security and pensions and all that jazz. Not so much, anymore. Now I think working at a large business is just as risky as working at a mid-size business. The risk one place is you'll get replaced -- the risk at the other is the business will go belly up. I tend to think you've got more control over your own destiny at the smaller shops. :)