When using Firefox on Windows to download the newest version of Adobe Flash Player, users are shown the following page (October 2010):
As you can see on the right hand side of the page, the checkbox to download McAfee “Security Scan plus” is pre-selected, meaning that a certain percentage of unsuspecting users will download this software and install it without meaning to. McAfee Security Scan Plus has not yet been reviewed by any Darkpatterns.org contributors [verification needed], but the app seems to scan your system and if it discovers you do not have a firewall or virus software (etc), it suggests that you buy a related McAfee product. Rather than providing any actual functionality, it simply appears to be a sales device.
The above public tweet from Adobe Employee @jdowell makes it clear why the McAfee checkbox is pre-selected. McAfee are paying customers, and they apparently demand it to be designed this way.
In another twitter conversation, @jdowell refers to the impact on conversion rates that an (ethical) opt-in design would have. (Thanks to @seb_ly and @plo for this example: further details on his blog)