I have experienced some issues with EXT4 and data losses too, but more extreme than you all describe. I installed jaunty alpha 3 two days ago and have all the updates installed. Since the install 2 days ago I have lost data on 3 occations. The most strange losses is:
* the computer wiped out a whole network share mounted in fstab
* the computer one time also removed ~/.icons when I empty the trash
The data losses never happened after a crasch or power failure.