Hmm, great question.
I suppose if I was told in enough time to take anything and was able to drive away in my car, I probably would (once I get a sewing machine, which will be sometime this year). Then again, I like the answer about insurance buying me a new one, so it would depend on whether the thing I'm evacuating for is covered by insurance. (Natural disasters often aren't covered.)
I keep a folder of my most important papers, such as my birth certificate, so that would come with me, along with my purse and laptop. The desktop would come, too, because it's pre-Vista Windows and with a monitor that isn't one of the wide ones. I wouldn't be able to replace it easily. Then there are some books I'd insist on bringing, and I suppose I ought to take some clothes. But LOL, I'd take my comfy clothes, not what I wear to work.