On the redesign side of the news, the new look is cleaner, faster to load, and introduces a whole new way of browsing events which centers around more specific metro's and integrated maps. The new interface is overall more media driven, and resembles in a way the recent Facebook redesign.
In addition to the most obvious changes mentioned above, Upcoming.org is no longer actually upcoming.org, but upcoming.yahoo.com.
Update: Apparently the updates were not unannounced; they were posted on the official blog multiple times. They did not, however, send an email notice to users in the same way Flickr did.
