Is there anything that lets you "reflow" PDFs, so as to make them more readable on Kindles?

Only in the sense you can convert PDFs to native format through your Amazon account.

It works fine when you have a clean PDF with nothing but serial text (e.g. a novel).

Completely useless if your document is a less than perfect OCR job, if it has reasonably complex tables, math or non-Roman text (especially laid out in parallel with English translation). In other words, there's basically no way to read physics or Chinese philosophy papers from JSTOR with it.

Your average programming book might make it through okay, but might not. At the very least it's going to kill the indentation of any code samples (very bad for Python books).

Yes, Amazon will do that conversion for you. It works OK, but I gave up on doing it for science white papers. Graphics are lost and it only works if the PDF is formatted the right way, so it was enough of a pain that I stuck to printing stuff out.

I use Readability to take web pages, get rid of pagination and beam the raw text to my Kindle, and I love it.

yes, k2pdfopt does a great job of making PDFs readable on Kindles, etc.