Andrew Morton [interview] posted on the lkml, "In 2.4.20-pre5 an optimisation was made to the ext3 fsync function which can very easily cause file data corruption at unmount time". This bug only affects people using ext3 in the uncommon "data=journal" mode, or files operating under "chattr -j", and does not affect the 2.5 series of kernels.
Andrew went on to say that "The symptoms are that any file data which was written within the thirty seconds prior to the unmount may not make it to disk. A workaround is to run `sync' before unmounting". He also posted a patch to fix the problem. However, soon thereafter, he posted saying that "that 'fix' didn't fix it. Sorry about that". Until a proper fix can be developed, he recommends that people "please avoid ext3/data=journal". Since "data=journal" is not the default ext3 mode, it is unlikely most people running ext3 will be affected by this. However, it is a data corruption bug so you should double-check that you use either "data=ordered" or "data=writeback" as your ext3 mode of operation.