From Enid Burns at ClickZ:

Users who block cookies are causing problems overlooked in the recent alarm over cookie deletion. A report published by WebTrends details which industries are most affected by users who refuse to accept their Web site cookies.

Over the 16-month period beginning January 2004, and ending in April 2005, overall cookie rejection rate rose from 2.84 percent to 12.4 percent. This sharp increase is credited to software that blocks cookies, as well as the release of Windows XP Service Pack 2 and evolving firewall settings. A recent report published by independent analyst firm, JupiterResearch, supports the growth of cookie deletion; citing 28 percent of users as claiming they selectively reject third-party cookies.