Well, it turns out that the Sony Reader settings work just fine for the upgraded Kindle 2, since the screens of the two machines are the same size. You can start from there and change the font size and style.
The cost? $0. The only catch is that you have to register as a Feedbooks member, as far as I know.
Manybooks.net offers similar custom PDF capabilities, and I suspect they should work as well.
Alas, this won’t help us with, say, DRMed bestsellers in any format—Amazon still needs to improve its font capabilities and add optional all-text bolding, and Feedbooks and Manybooks are a long way from carrying everything. But this is better than no solution if you dislike the Kindle’s own font and are willing to suffer the limits of PDF on the machine. No text to speech, alas, although, it turns out you can word search (thanks, GJN).
Yet another catch is that the Kindle’s built-in browser apparently won’t download PDF book’s directly. Another fix for Amazon to make? I copied the shown PDF of David Copperfield from my Acer netbook.
In case you’re curious, I took the screenshot by pressing alt-shift-G and copying the resultant file to the netbook, on which I’m typing out this post. The text-background contrast unfortunately is lower than on the actual Kindle.