How many of us take a pause to think about the differences between ext3 and ext2 filesystems? I would guess not many. It has been all around acknowledged that ext3 is superior to ext2 and most Linux distributions default to formating ones partition to ext3 unless specified otherwise. If you ask me, one of the fundamental differences between ext2 and ext3 is that ext3 has journaling support inbuilt into it where as ext2 does not.
Now here is the interesting part; if you do a clean unmount of the ext3 file system, there is nothing to stop you from mounting it as a ext2 filesystem and your system will recognise the partition as well as the data in it with out any problem.
I came across this excellent analysis of ext3 journaling filesystem by Dr. Steven Tweedie which should be an informative read for people interested in the subject of filesystems. The article was writen in 2000 but I believe what has been explained there has not lost its relevence even today.