"A window is transparent but a door is open, we're trying to be like a door," Evan said.
A door that will hopefully add value (which in these specific cases means traffic/sales) to the media properties, search engines and retail outlets that Twitter hopes will adopt the service.
From the Twitter blog:
When we're ready to launch, initial participating sites will
include Amazon, AdAge, Bing, Citysearch, Digg, eBay, The Huffington
Post, Meebo, MSNBC.com, The New York Times, Salesforce.com, Yahoo!, and
YouTube. Imagine being able to follow a New York Times journalist
directly from her byline, tweet about a video without leaving YouTube,
and discover new Twitter accounts while visiting the Yahoo! home
page--and that's just the beginning. Twitter has proven to be compelling
in a variety of ways. With @anywhere, web site owners and operators
will be able to offer visitors more value with less heavy lifting.