The New York Times article Start-Ups Follow Twitter, and Become Neighbors covers a group of companies that have purposely rented space in Twitter's building.
The key quote from the article is:
“Even though it’s all about tech and the Internet, the real magic of Silicon Valley comes from people being in the same space,” said Burt Herman, co-founder of Storify.
If you read the article, you will see that the reasons these firms are interested in co-locating with Twitter mostly fall under the category of "accelerated serendipity."
The implications for coworking are pretty obvious.
We cover this and the broader new localism trend in more detail over at Smallbizlabs.