It’s an inside joke for multi-time WWDC attendees to watch the first-timers show up to registration in shorts and a t-shirt. The newcomers are thinking “California in the summer. No need for jeans and layers.” They quickly learn the value of a jacket, thanks in large part to Apple who hands out a free one to each attendee during registration. If you forgot to pack one, you can join the thousands of other look-a-likes on Market Street proudly sporting the current year on their backs.
With the move to San Jose, which does actually participate in California summers, one has to wonder if jackets will still be the swag of WWDC. Apple has handed out laptop bags in previous years, so they are a strong contender. On the other hand, air-conditioned convention halls are always frigid, and San Jose nights are cool enough for jackets, so it still might make sense to hand out jackets.
My money is on the laptop bags.