Isn't this kindof given on the west coast? I can't imagine a single hacker I know wearing a suit (and meaning it).
With the exception of the very small companies, every company where I've worked has had at least one hacker who wore a suit.
The suit-wearing hackers may outlast the lumber-hacker look.
It's kind of obvious when the guy on the other side of the table is wearing shorts and sandals that perhaps a suit and tie is inappropriate...
For instance, wearing a scruffy t-shirt and jeans with the suit to signal non-pretentiousness?
Personally, I enjoy wearing suits but if I ever go to Silicon Valley, I would like to avoid committing a faux pas :)