According to AllThingsD, this puts the valuation for The Huffington Post at 100 million dollars. It's a big valuation, and the 25 million dollar round is bigger than the previously rumored 15 million. A question that inevitably still pops up after any news such as this is: "How does this make sense? It's just a blog!" I, however, don't see why not.
I've checked out Huffington Post - which I rarely normally do as I'm not interested in politics - and I don't see any crucial differences between this site, and any big media publication on the web. They've got news, reports, features, they seem to be timely and thorough, and they produce a lot of content. On the internet, that's pretty much all that matters: whether you've got a team of 100 people behind the site or it's just a one man gig, all your readers care about is the content.
The only difference between a blog and any other media publication is that blogs scale very well. Traditionally, if you want to start a newspaper, you need editors, journalists, graphic designers, you need money for print, you need marketing, and a zillion other things just to start off. And it all costs a lot of money.