Just the notes, no download yet:
ReleaseNotes - k9mail - Changes in each revision of K-9 - Project Hosting on Google Code
Looks like a lot of bug fixes, maturing, and some new features. The feature I'm looking forward to:
Implements new setting "Sync remote deletions" which is checked by default. When unchecked, K-9 Mail will no longer remove messages from local storage just because the message was removed from the server. This functionality works for all account types. Messages will still be removed from the local store in order to stay within the limit imposed by the "Number of messages to display" setting.