Version:           3.5.6 (using KDE 3.5.6, Kubuntu (feisty) 4:3.5.6-0ubuntu14)
Compiler:          Target: i486-linux-gnu
OS:                Linux (i686) release 2.6.20.3-ubuntu1-wo-usbsuspend-ppdev

For a quick search in several folders, I'd prefer if the search bar was not cleared when switching to another folder, but instead the search was automatically updated with the new folder's matching contents. 
This would allow continue searching in other folders when a search did not yield the desired results. I can always clear the search field manually to end my search when I want to, but currently KMail's behaviour is IMO unnecessary limiting if not annoying.