I used "Legacy Filevault" before it was legacy. Then, when I upgraded to Lion, it took some additional months for me to get around to the FDE upgrade. I had to move around a lot of data to make room.
I consider this a pretty big deal.
I just checked and, sure enough, my cleartext password is visible if I run:
$ sudo cat /var/log/asl/* | strings | grep 'password ='
Guess today's a good day to switch to FDE
I stand by (most of) my original comment.
Evidently not!
There was a workaround I had running for a while in a large deployment in the form of a logouthook that would copy the relevant prefs to a location that LaunchServices would find it before FileVault finished mounting the home directory.
There were a lot of corporate/education deployments running "legacy" FileVault who hadn't invested in PGP WDE or other commercial options, and I bet they haven't all upgraded instantly to the new FileVault when they moved to Lion.
Which means nothing at all, especially since hackers are not known for their extended social circles. It's also the dictionary definition of "anecdotal evidence".
If you want to gauge how many people used the old FileVault, well, one of the ways would be to compare old FileVault support mailing list traffic to the traffic for lists of other OS X offerings.