Szymon Stefanek's KMail GSoC project was merged into trunk today.

Stunning. You've got to try it. KMail has been re-launched!

r881321

To quote the commit message:
"... New features include grouping, multiline items and the ability to customize just about everything of the look of the message list.
Also, we have a tabbed interface for opening more than one folder at once now..."

Fantastic work Szymon and Thomas.