I'm sorry, I don't get it. Or rather, I'm sorta disappointed.
As Kottke says in his quote in the Wikipedia definition of Tumblelog (jees never thought I'd quote Kottke!), this just seems to me like what Weblogs were originally anyways: a place to quickly dump thoughts and stuff you found on the web, on a webpage, be it for personal or sharing purposes.
They claim they are less structured but if anything I see them as more structured. Or rather "strongly typed", as some programming languages are: you need to define content types and assign your postings to them (or I suppose there is some auto-detection built in), and not doing so will break something. Considering the stern "technological determinism" sermon I got from Lenczner when I told him file type is an inherent, important and immutable quality of a digital object ("why should I treat this piece of media differently just because of what race it is?!"), I must scratch my chin over all this.
So, I must apologize to Bosko and the audience at BarCamp Montreal for asking a question that must have seemed totally out of leftfield: Bosko was talking about tumbleblogs as digital scrapbooks and I was thinking totally on the frontend of things (my "Breaking out of the Document" post a few days ago was precisely about this, sparked by an exchange with Julian Bleeker about a finding a way to produce truly scrapbook-like presentations on the web, of gathered (tumbled?) content, be it aggregated/reblogged bits, bookmarks (URI), audio/video/photo objects etc...)... and Bosko had been talking about the REST architecture of his tumbleblog service. Woops! ;)
I'm not saying it isn't interesting, just that it isn't at all what I thought it was, and while I recognize I may be missing something, I don't see how tumbleblogs aren't just a further formalization of "weblogs."