Solaris 10 needs an SMI (VTOC) label on the root pool disk. This is a long-standing boot limitation.
Convert the disk label to SMI by using format -e. Watch that it doesn't put on a default label with 128MB in slice 0.
If it does, move the disk space back to s0.
See these pointers:
Thanks, I realized I wasn't doing an All Free Hog partition and not using slice 0.