I grepped the 2.6.24 sources, fs/ext3/*.c and fs/jbd/*.c, for any mention of checksum, and couldn't find it. However the email lists do have some reference to a journal checksum patch for ext3 that didn't make it into 2.6.25.
One other thought: perhaps LVM is bad for data integrity with ext3 because, as well as stopping barriers from working, LVM generates more fragmentation in the ext3 journal - that's one of the conditions mentioned by Ted Tso as potentially causing write reordering and hence FS corruption here: http://linux.derkeiler.com/Mailing-Lists/Kernel/2008-05/m...