Section 9 of ICANN's press release makes it clear that the SO's are to be
legally separate from ICANN. It is therefore pretty much a given that
ICANN's membership will not be the same as the DNSO's (although there will
likely be significant overlap). If the two memberships were the same, there
would be no reason for separate SO's, the DNSO would be ICANN.
This also impacts the incorporation question. ICANN has now made it clear
that "ICANN's liability insurance and other corporate resources will not be
available to Supporting Organizations." The SO's options are either to
incorporate or to remain unincorporated associations without legal
protection for their members. IANAL, but the choice looks obvious to me.