Just a quick post. In Kubuntu 11.10, running KDE 4.7.2 (and possibly earlier) I have noticed a problem. When using the File-Choosing dialog, I click enable Aside Preivew, which works for the time. Once I choose a file and the dialog closes, if I reopen the File Chooser, the preview option is disabled again and I have to manually re-enable it.
This is a quick fix for force enabling Previews in QT/KDE's File Chooser (KFileDialog).
Open the configuration file '~/.kde/share/config/konquerorrc'. Find the section marked '[KFileDialog Settings]'. Find the line which reads "Previews=false" and change it to "Previews=true".
Save the file and exit it. Restart whatever program you want this to take effect it, and Aside Previews should be enabled by default.
IF YOU NEED HELP EDITING CONFIGURATION FILES UNDER LINUX, please see this link: http://awesomelinux.blogspot.com/2010/09/how-to-edit-configuration-file.html