Version:           1.9.1 (using KDE KDE 3.5.5)
Installed from:    Gentoo Packages
Compiler:          Portage 2.1.2_rc4-r1 (default-linux/x86/2006.0, gcc-4.1.1, glibc-2.4-r4, 2.6.12-gentoo-r6 i686) 
OS:                Linux

Steps:
1. filtered a folder using the "search field" for the term "rate your stay". Folder has 11139 messages. One message subject contained the search term, the list of messages was reduced to a single line.
2. With the mouse dragged&dropped the message on an other folder. A small widow came, asking Copy/move/cancel. I selected move. One processor load jumped up to 100%.
3. While the processor still busy I clicked on the icon of a third folder, to make kmail switch to that folder after performing the move operation.
4. After a few seconds kmail crashed. After restarting kmail I found that the message was not deleted from the folder of origin and was not present in the target folder.

Reproducubility:
a) tried twice in the above described way, crashed both times. I will attach both tracebacks.
b) omitting step 3, in other words not cliking on an other folder while kmail busy - the move operation was performed without a crash