Twitter founders Evan William and Biz Stone visited YCombinator’s Startup School this Saturday, and apparently had a productive kibbitz session with the YC team, who suddenly decided to extend the deadline for its 2010 winter session by two days to allow for more Tweet-centric startups. The deadline's been moved from 10pm Monday night to the same time Wednesday. Startups that apply to build services on top of Twitter will get priority access to the Twitter stream and to people, presumably Ev and Biz, at Twitter. YC’s Paul Graham explained the thinking behind the new partnership in a post on Monday, likening the microblogging platform to the fundamental buildling blocks of the web:

"Twitter is important because it's a new protocol. Fundamentally it's a messaging protocol where you don't specify the recipients. It's really more of a discovery than an invention; that square was always there in the periodic table of protocols, but no one had quite hit it squarely.

"Successful new protocols are rare. There are only a handful of commonly used ones: TCP/IP (the Internet), SMTP (email), HTTP (the web), and so on. So any new protocol is a big deal. Each one of those protocols has spawned many successful companies. Twitter will too."

The extended deadline also applies to startups building on top of life-casting platform Justin.tv, though that partnership has been contemplated for a while, according to the announcement. Both initiatives reflect a focus on real-time data platforms, which many angel investors like Ron Conway believe is The Next Big Thing.

There are, of course, a swath of startups already based on the Twitter API, (TweetDeck, Tweecha, Echofon, etc.) but priority access to the Twitter stream and personnel could differentiate the companies coming out of the YC deal.

Earlier this year, YC began issuing “Requests for Startups” (RFSes) that specify an idea the incubator believes is ripe for early-stage startups. Since the first RFS in August, YC has issued four, which together constitute a pretty good summary of fertile startup territory. The Twitter and Justin.tv initiatives are the third and fourth RFSes, respectively; the first two highlight the future of journalism and new paths from consumer to vendor.