Went to a seminar yesterday for people wanting to set up their own business.
The chap running it told us that as of 1 February, those who are self-employed have the option to pay into the Rentenversicherung for 40 Euros a month (this is a flat rate, regardless of income).
If your business goes bust, you can then claim unemployment benefit. The payments are based on a scale based on your level of education. So, for example, someone with a doctorate and a child could get 1,300 euro per month.
Arbeitsamt Info leaflet (PDF file) here
We all hope that it won't happen - but it is good to have a safety net.
You'll find more info if you google "freiwillige rentenversicherung"