I don’t really see what has changed, but I guess the guys over at Twitter felt it was a good time to revisit the Twitter terms of service.  

One noticeable thing, which I think most people already took for granted, is that Twitter users own their Tweets.  The owners are also still keeping the doors open on advertising.  There is no news yet on what they may do to monetize the site, but the TOS reads:  

The Services may include advertisements, which may be targeted to the Content or information on the Services, queries made through the Services, or other information. The types and extent of advertising by Twitter on the Services are subject to change. In consideration for Twitter granting you access to and use of the Services, you agree that Twitter and its third party providers and partners may place such advertising on the Services or in connection with the display of Content or information from the Services whether submitted by you or others.

It is my guess that if Twitter does take the advertising route, they will make large overall deals with premium CPMs, but then again, what do I know?  They could create their own contextually related and super-targeted ad platform similar to Adsense.  

The thing that Twitter can really bank on (and sell) is the ability to watch conversations as they happen and know what people are talking about (ie, brands, companies, products, movies, culture) right NOW.

To marketers, advertisers, and companies, this is very valuable, and not only because Twitter has captured the public’s imagination and can be quite useful, but because it might be a great way to promote a business.