Version: (using KDE 4.4.2) Installed from: Ubuntu Packages Virtualbox hard-drive images (.vdi files) are often many Gigabytes. Because they are considered to be "plain-text" files by KDE/Dolphin, they are associated with Kate by default. Accidentally clicking such a file in Dolphin caused Kate to attempt to load the file resulting in a completely unresponsive system. A better solution would be to either associate the files with Virtualbox, or to not give them any initial association.
shared-mime-info does not have an entry for vdi files, from what I can see. You could file an enhancement request to add this at http://bugs.freedesktop.org/
I guess that's not the correct way but as the mimetype configuration allows to add mimetype definitions I created my own "application/virtualbox-vdi" one with an icon that shows a harddisk on a sheet.