This was asked on the blog recently too. I always try to update the readme.txt with the big new features, but until now that file has been designed to make sense when upgrading from stable to stable - something that happens to rarely I guess.
I should probably do something more obvious for the testing builds, but until then you can view the changes one of two ways:
1. by change number (the last part of the build). The current release is 970, and the last was 962 so you can see what was checked in between those revisions here:
http://code.google.com/p/pnotepad/source/list?start=970
2. by bug changes:
http://code.google.com/p/pnotepad/issues/list?can=1&q=status:Fixed&sort=-closed+-id&colspec=ID%20Type%20Status%20Priority%20Milestone%20Owner%20Summary%20Closed
Whatever's been closed between last release date and current release date are the issues that have been resolved in that release. This list is less clear because it will include duplicates, won't fix items and other non-fixes. There are also some changes that don't have related issues, although that's unusual.
