Doh! While trying to get my slax distro onto a USB key, I fucked up big time and dos formatted (mkfs.vfat) my hardrive (my head is still in the era where hard disks are hdX, not sdX).
gpart can try to guess where your partitions were but it doesn't know LVM2
LVM2 partitions have a sector which begins with "LABELONE" as documented in these slides (thanks to agk on #lvm!). I invoked the following script with ./lvm.pl /dev/sda to get a list of all the sectors that contains that string.
Stuffing all those sector numbers into $ss and then doing