For KMail, the most work has gone into actually porting it to Qt4 and KDE4. This was a huge effort, since KMail and Kontact have a big codebase, and the porting resulted in many regressions. At the point of the KDE 4.0 release, there were simply too many of those regressions that made Kontact not very usable. We simply didn't have enough manpower to make a release happen at that time, and we could still use more developers to help with coding for the KDE 4.1 release!
We've come to a point where most of the regressions are fixed, and KMail is usable again now. I use it for my daily work with POP3, and don't have any major problems. IMAP also seems to work reasonably well.
The port to Qt4 is almost complete, the only major thing missing is the folder list and the message list, both of which will be taken care of by the SoC project by Szymon Stefanek, who also has exciting plans for a new, more modern, look of the message list.
For the KDE 4.1 release, I mainly plan to stabilize it more and continue fixing regressions. Even though most of the work has gone into porting, KMail will also contain new features. Some I remember off the top of my head are" support for tagging, support for adding links to HTML messages, and some spell checking improvements.
A major new feature of KMail is that it will now also run on Windows and MacOS X, thanks to Jaroslaw Staniek who worked hard on the Windows port. Below you'll see a screenshot of Kontact running on Windows.
Because there were some problems with the way KMail stores its index information, KMail now uses SQLite for that on Windows.
The long term plans for KMail is of course porting it to Akonadi, which will require huge changes to the code, but will finally replace the monolithic and non-crossplatform storage layer of KMail. This will take some time, it is unlikely to happen before KDE 4.2.