From
http://ycombinator.com/rfs3.html (the Y Combinator requests for startups):
"We want to fund those companies. And the people at Twitter also want to encourage people to build stuff on top of it. So together we came up with a plan: anyone YC funds to do a startup based on Twitter will get priority access to the Twitter stream, and to people at Twitter."
Yet HN seems to be filled with stories of people who built something on Twitter only to have their API access pulled. Granted, most of them are going against the TOS and basically building a Twitter client, but I frequently hear the advice to "not build a startup on top of Twitter."
Note: I'm particularly curious because the startup my co-founder and I are building, while not 100% Twitter-based, would be absolutely revolutionized were it given full Twitter stream/API access