In our area it is quite common. Does not affect the value. For my knowledge it does not affect your health as long as it is covered and not disturbed.
As others stated if removing it is your budget go for it. Otherwise not and just disclose it. Many improvements offer a good return when selling. This one does not.
Lee Ginsburg -Prudential. http://www.leesellsmore.com
You should contact a company that specializing in removing and reinstalling new air ducts.
We are in Los Angeles and the company I used for our "old" house was Indoor Air Care.
They did a great job all around with great price and great tech's.
Hope this helps.
Your agent should have mentioned asbestos when you bought the house but since he/she didn't, be aware of it in the future. You can never over disclose any known fact about your house to potential sellers. EVER.
If you do the work now, you can tell a buyer that you have done the asbestos work with certified guys.
As far as asbestos, it only takes a little bit, anyway. It was in hair dryers, too, and school tiles. It's probably more than a little bit, if it was used for insulation. What? Insulate just a little bit of the ducts?
Material defects..disclose, disclose.