Umair Haque : What's really disruptive?
I'd suggest it's better to stick to "disruptive" in the Clayton Christensen definition, as something the incumbents can't get into because it's worse (or useless) from the perspective of their existing customers.
In this sense, I see Ning as being genuinely disruptive, if you consider it as a web-development platform in competition with Microsoft's Visual Studio, IIS, database products etc. Or with similar Java based web-middleware from Sun, or even with Ruby on Rails and other free offerings.
Although Ning is "worse" in the sense that it builds a limited range of applications, it might be able to build the applications most people want. But MS or Sun couldn't get into it without abandoning their existing developer customers who have more sophisticated requirements and are already commited to their own existing codebases.
If I was running Ning, I'd be adding a few rather bread-and-butter useful apps like weblogs, discussion forums, issue-trackers etc. And aggressively selling it (with training courses, online documentation, screencasts etc.) to small web-development agencies as an alternative technology for building stuff for their clients.
I'd charge these small development agencies for an advanced product that allowed them to more fully wrap the applications in their client's branding, for hosting, and for the ability to "compile" Ning apps. into something that could be taken away and hosted elsewhere.
Ning has the potential to disrupt Microsoft, Sun, IBM and everyone providing web-based / enterprise software. And a great deal of the free toolkits as well. (Personal note, can we have Python-Ning as well as PHP?)
A web-centric development platform is the necessary requisite for the web-as-platform, and Ning has a chance of being it.
Update : as mentioned before. The one giant who doesn't have a current stake in web development platforms, and is therefore a good match for Ning, is Google.