According to Wikipedia : 
“When the mail user agent process finds messages in the new directory it moves them to cur (using rename() - link then unlink strategy may result in having the message duplicated) and appends an informational suffix to the filename before reading them.”

So I understand that I shouldn't see any file in the new directories. However I have a few ones:
$ find ~/.kde/share/apps/kmail/mail/ -type f | grep -c '/new/'
3902

They are not duplicated messages. Meaning that these messages, as far as I can see, are not in the cur/ directory. One of them, for instance, is new/1342795373.R533.basilic:2,S and we can see that the informational suffix has been appended. I can also see this message in kmail and it cannot be distinguished from other messages (in the same folder in kmail) that are stored in the cur/ directory.

I'm definitely not a maildir expert but that seems to be a non-standard behaviour. I'm also wondering if this can explain the following bug I filed several months ago : https://bugs.kde.org/show_bug.cgi?id=289657

Reproducible: Sometimes

Steps to Reproduce:
Most messages are put in cur/ directories. I don't know what prevent some of them to go there!