Question for Joel: How important is it to stick to one industry?

Early in my career, a family friend who's been quite successful (started as a nurse, advanced into Administration and is now consulting) told me to always build on my career rather than jumping around from industry to industry and job to job. Her advice rings in my ears to this day but I have to wonder if it is still the right advice.

At what point should someone stick with the industry they are in? What advice would you give to someone who has been in their industry for at least 10 years but doesn't like it? Or to someone being laid off from one industry and considering something new? Is it harder for someone with experience in one industry to change to another?