Just reporting some observations after making a brand new installation of Ubuntu 9.04 with ext4 as default file system on my Sony Vaio VGN-FS195VP. Since the installation some days ago I had again four hard locks, but luckily - despite my experiences some weeks ago - without any data loss. All of them happened with the standard installation of Ubuntu on the Gnome desktop.
One hard lock happened when listening internet radio with quodlibet in the background and trying to update Ubuntu via Synaptics. Another one when trying to rip a dvd with wine and dvdshrink in the background and trying to open other applications (Firefox, Bluefish, gFTP) almost at the same time. The other two happended when trying to remove some ISO files of DVDs (together maybe about 12 GB data) from the trash. The trash icon on the desktop turned empty (actuallly a good sign), but about five seconds later the entire system crashed .
The Kernel running on my system is 2.6.28-11-generic and the Gnome version is 2.6.28-11-generic. Since I am not a very technical guy, I have not applied any of the above mentioned remedies. But as I am very happy with ext4 as my default file system (and have not yet experienced data loss!) I will keep it hoping that there will be some fixes in the next Kernel.
Thanks for all your explanations about ext4, Theodore Ts'o, and keep up the good work!