I also have backed away from Thunderbird and onto KMail because of the lack of maildir support. It’s a pity, because I support many Windows-based systems and the ideal ‘persuader’ for getting people off Windows and onto Linux is having identical apps in both environments….. FireFox and OpenOffice.org are doing a great job in this respect, but Thunderbird stands out as not-quite-there yet – actually the maildir support is more important for the Windows environment particularly because of the antivirus requirements and the ability to quarantine individual infected mails rather than the whole jolly inbox file.