First off, I do not believe the real-time web is only about new content that's published quickly, in response to real-world developments.
I believe the real-time web is more appropriately defined as web-accessible material that people are interested in, right now. In this sense, the real-time web is really more about the "attention frontier" of people on the web, rather than a technical timing property of the underlying material itself.
Of course, when significant events unfold in the real world, new material is created on the web that describes what's happening, and people are interested in that new material.
Take, for example, Michael Jackson's unfortunate demise. At the time of his death, and for a significant amount of time thereafter, people were interested in updates on what might have killed him, but they were also interested in his songs and videos - classics such as "Thriller" were watched in significant volume on YouTube. Those videos, while not in any sense 'new', were still part of the real-time web at that moment in time.
So I feel that the real-time web is really about this: the information that people are looking for, right now, on the web.
So what about Twitter? Does Twitter define the real-time web, since tweets are being issued in real-time? I don't think so. Twitter is a fantastically valuable planet-wide alert system. It's a great way of signing up for alerts from people and organizations that you care about. It is not, however, the same thing as the real-time web. There is so much more content that needs to be accounted for, such as blogs, news, corporate pages, etc - in fact, any public website that creates new content is potentially part of the real-time web.
I view Twitter as a (large, disconnected) conversation. Some of Twitter's subject matter is Twitter itself. It can be rather inward looking, and hash tags such as #FollowFriday and #MusicMonday are a syntactic manifestation of that.
But some part of Twitter is a conversation about the web. Approximately 20% of tweets contain URLs. I view such tweets as comments about the web itself. After an appropriate filtering mechanism (based on re-tweet and follower counts) these URL-bearing tweets can be "de-referenced" and used to form part of a picture about what people are interested in, on the web, at any given point in time. But the picture that one obtains from Twitter is partial. Other sources of data are needed to form a complete picture of the real-time web.
So, as a marketer, the real-time web is valuable as an indication of what your potential customers (and detractors!) are interested in, right now. If you can reach these people with an appropriate message about your product or service at the moment of their greatest interest, that's clearly of significant value - to you and to them.
There's an emerging crop of real-time information services that try to deliver a picture of the real-time web (one is Wowd, the company for which I work).
Some are based on the idea of discovery - that is, showing people new material that's extracted from the overall attention frontier at the moment.
Some are based on search - that is, helping people find fresh, quality information resources on a topic in which they already have some interest.
Others are focused more on recommendation - that is, on making specific recommendations for information to consider, based on real-time correlations between one person's interests and the interests of other people who are similar to them in some way.
Each of these services provides a different opportunity, to a marketer, to get a relevant, timely message in front of a user.
The goal of course is to deliver a timely message in a way that topically connects to the user's current task, and in a way that doesn't interrupt what the user is currently doing.
Those twin principles of topical connection and non-interruption are the legs upon which search advertising walks... finding the best way to instantiate these same principles in non-search-based user-engagement mechanisms (discovery & recommendation) is an on-going exercise for those of us deeply involved in the real-time web.