We should all be pleased that the five big search engines have now shown a commitment to reducing the length of time that they retain potentially sensitive data about their users. Microsoft and Yahoo! announced their intentions this week, which means that Microsoft and Google have publicly agreed to keep user data for no longer than 18 months, Yahoo! and AOL for 13 months and Ask.com is unveiling a new tool that allows users to block the retention of specific search terms and their computer's IP address. Does this suggest that for the serious privacy advocate there's really only one…