Yup they do expire. Sorry to say. I'm not sure if the same holds true for the Net+, but A+ definately does expire, in fact they print the expiration date on your certificate when they mail it to you. I think the old old ones (like back in 2000) didn't expire, but they realized that hardware changes so often that they need to update the exam at least every three years. After making that realization I believe they made the decision to make certifications expire after so many years.
I mean think about it as a company. Would you want someone toating around saying they were certified by your company when in fact they were certified in 98 based off technology back then? Hardware changes entirely too often for someone to take an exam and be able to say 'i'm certified for life.' I know my MCP exam is the same way. I'm certified in an OS technology and once Microsoft stops supporting that OS the certification is no longer valid.