Funny you should mention this...The Startup Foundry is doing exactly that. A blog focused only on startups. Check it out, we have gained a lot of traction and have a relatively large following for being so new (about two weeks). http://thestartupfoundry.com
Let's say you cover the next Twitter or Facebook before they become the next Twitter or Facebook. Once they explode in popularity, are you going to stop writing about them? Even though those articles are getting the most traffic on the site? And tons of speciality sites start appearing just about Twitter and Facebook. And you even know people from Twitter and Facebook since you profiled them when they were little, and can get the inside scoop on changes and upcoming features.