For testing I installed ext4 together with Jaunty Alpha 4 as standard root file system on my Sony Vaio. Since then I had four hardlocks, two of them completely destroying my gnome desktop. So far, this only happens within Gnome while upgrading the system with apt-get in a shell AND at the same time running and working with other programms (like quodlibet, firefox, Thunderbird, Bluefish etc.).
As of the gnome desktop destructions, in one case apt-get unfortunately was just installing some xorg-server files and - in the other case - configuring the gnome-desktop, when the hard lock happened. The sad part is that I didn't find a way to repair the broken system neither with apt-get nor with dpkg nor with aptitute as the size some needed configuration files was set to zero by the crash. So, for now I am switching back to ext4 releasing this warning:
DON'T DO ANYTHING WHILE UPGRADING UBUNTU WITH EXT4!