I think this is a better idea than relying on advertising, but do you think there'd be enough demand for a white label version for it to be the core of their model?

Another approach: Fred Wilson's post today on comments constituting blog posts suggests that maybe they're building towards a sort of blogging platform for non-bloggers, where the aggregate of a user's comments on other blogs would form a new low-maintenance blog (on the Disqus site).

That's an interesting thought. At first, I was thinking what you were suggesting would be very Tumblr-like, but Disqus would centralize your comments elsewhere, and maybe you could opt to pull a subset in to make your new platform?

Precisely. Then add simple tools for categorisation, design customisation and so forth.

The point is that there are probably 10 active commentators for each active blogger. Perhaps Disqus is building towards a platform for people like this, who comment actively but who don't want or think they wouldn't be able to run their own blog.