Valleywag Misses the Mark on Y Combinator
onwebapps.com
onwebapps.com
They're made to be acquisition targets.
Source? I think that's based on anecdotal evidence.
There are a number of startups generating revenues (reddit, loopt, TextPayMe, Inkling).
And a number of others that have a reasonable way to get revenue: (justin.tv - sell video service to celebrities; scribd - contextual advertising; shoutfit - sell trends to fashion houses; thinkature - sell collab software to businesses; octopart - sell product advertising; wufoo - sell upgraded forms that can be embedded into your website).
Come to think of it, I have a hard time pointing out specific YC companies that don't have any revenue model. Maybe there's a few - but the fast majority of them do have the potential to be self sufficient.