An undefined volume appears out of nowhere?
diskutil list will show you all the Lion core storage volumes and might be of use. The df command shows more network mounted volumes and will probably allow you to match that long string of characters to the storage structure used for MobileBackups in time machine. 

mac:~ mike$ diskutil list
/dev/disk0
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *500.1 GB   disk0
   1:                        EFI                         209.7 MB   disk0s1
   2:                  Apple_HFS mac                     499.2 GB   disk0s2
   3:                 Apple_Boot Recovery HD             650.0 MB   disk0s3
/dev/disk1
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:     Apple_partition_scheme                        *1.8 MB     disk1
   1:        Apple_partition_map                         32.3 KB    disk1s1
   2:                  Apple_HFS Google Chrome 17.0.9... 1.7 MB     disk1s2
mac:~ mike$ df
Filesystem                        512-blocks      Used Available Capacity  Mounted on
/dev/disk0s2                       975093952 837212344 137369608    86%    /
devfs                                    366       366         0   100%    /dev
map -hosts                                 0         0         0   100%    /net
map auto_home                              0         0         0   100%    /home
/dev/disk1s2                            3412      3412         0   100%    /private/tmp/UpdateEngine-mount.yzrVRXvfxf
localhost:/J91gqbWGFXawS6VmjrAKYp  975093952 975093952         0   100%    /Volumes/MobileBackups