That's what I have been told as well as what I found on the web. So if your radio or navigation system fails after your original 4 year/ 50k miles and you have the CPO warranty, it will not cover these items. I don't know if the extended warranty you can buy also doesn't cover these issues.
Official Items Not Covered By BMW CPO Warranty:
http://www.bmwusa.com/Standard/Conte.../Warranty.aspx