If I were a webmaster without a custom 404 page, I'd prefer this to users clicking a SERP over to IE's kiss-of-death 404 and a back button to Google and competitors. The webmaster gets 2 more chances, a site search, and it's back to Google land. Not saying they won't throw AdWords in there at some point. That wouldn't be very 'good' though.
Nonetheless, it appears as though the old non-custom 404 will become quite a rare find for users as every browser and toolbar out there tries to muscle-in their 404s like address bar search.