A survey made by Israel's Bureau of Statistics finds that within the general population in Israel, 87% of people having children at the age of 18 or less think that sites with inappropriate content should be blocked; However, less than half (43%) installed a blocking mechanism.
This seems like a good opportunity for ISP to provide a simple, network based, value-added service - with a monthly fee attached.
See "Selected Data from the 2010 Social Survey: Computer and Internet Usage" - here (Hebrew)
"[Israel]: ISPs will have to Offer Free Web Filtering Service" - here
"Kosher Mobile Internet - Cellcom Israel Offers Web Filtering" - here
In other countries -
Australia - "CyberOne [Australia] Uses ContentKeeper to Block Illegal Web Sites" - here
Turkey - "Vodafone Turkey Uses Optenet and Allot for Web Filtering" - here
UK - "Top 4 ISPs Committed for Opt-In Parental Control" - here
US: "Alltel Wireless Uses ByteMobile for Traffic Management and Parental Control" - here