Tim Peter's right that ad-supported widgets are micropayments on a couple of levels - content providers are paying distributors micropayments for impressions and clickthroughs, and then I'm paying as a consumer with a drain on my attention. I guess what I am fearing is a world where each widget provider offers free and paid versions, so as a consumer I'm not signed up for a paid version of a single start page (a la NetVibes) that's ad free, but rather I'm stuck making micropayments to multiple widgeteers if I want ad-free versions of their widgets on whatever start page. Yikes!
Until there are standards or semi-standards for how to produce Ajax widgets that will work on a variety of start pages, this could be the grim future.
Rich Campoamor is right on about the JSR 168 portlet spec. In the original spec (I don't know where it ended up), all of the meaty UI specs (such as standard CSS style names that would let you add a measure of coherence to an aggregated portlet start page) were an appendix and an afterthought to the spec. I'm not optimistic about Ajax widget interop... but like I said I'm too cynical.