Note the swap of disk1 and disk2 through reboots.
Sure, but this is not unexpected behavior, since (I presume) neither of these disks contains the startup volume.
Browsing the web, this seems to be mentioned in reference to the MacPro platform more than anything else (Probably because of the multiple drive bays).
This is only somewhat related to the fact that the Pro has internal bays -- specifically it is because they have integral SATA bus connections that typically (but not always) initialize connected drives faster at startup than external bus-connected ones like USB or Firewire.
The normal startup sequence (IOW, booting normally into Mac OS X) first attempts to initialize the drive with the volume on it designated in PRAM as the startup volume, which becomes disk0, then polls for any other available drives. They are assigned device ID strings & nodes in the order they become available. This insures the fastest possible startup because in some configurations the OS might be using a different drive for VM, User folders, or whatever. This not specific to the Mac Pro; in this respect all modern Macs are the same.
If you refer to the "DEVICES" section of the man page for diskutil, you will note that there are four ways provided to refer to a disk partition. The first two (string & node ID's) are never reliably constant & unchanging (because of the above). The volume mount point is constant (as long as you don't change the path name) & the Universally Unique Identifier (UUID), as the name suggests, survives even a path name change.
For this reason, the UUID the preferred way to refer to a volume when an unchanging reference is required. For instance, note from the previously mentioned link about using the fstab file (which is not the same thing as the fstab.hd file, which the man page for fstab should make clear), the file reference is in the UUID form. This is the form you should use too, at least where unchanging references are what you need.
