From the "what are people actually using their browsers for" files:

Microsoft's upcoming Internet Explorer 8 (IE 8) may get a private browsing (aka "porn mode) feature.  IE 8 Beta 2 is supposed to be out soon and a pair of newly filed Microsoft trademarks (pointed out by blogger Long Zheng) might be indications that the private browsing feature is coming.

What private browsing does is disable history and caching for a given session such that a user's browsing session is private. Apple Safari has this feature as an option today.

Reality is of course as anyone (not me) that has looked at inappropriate sites knows well is that since the very first Netscape browser you've been able to delete history after a session. Which is sort of a manual private mode. Firefox 3 has an option that lets you delete history and cookies immediately at the end of a session or on session start up (semi-automatic private mode?).

Users behind corporate firewalls however still wouldn't be safe though from snoopers as gateway appliances can still potentially track, identify and block users access to various sites.