I recall that same wording in previous editions of the web site. I did find this today on the FAQ section:

Q: Are past guests eligible for any discounts?

A: Seven Seas Society members, or guests who have sailed with us before, are eligible for a 5 to 10% savings on all voyages (policy valid through May 15, 2005). Effective May 16, 2005 members will be eligible for a 5 or 10% savings on select voyages. Restrictions apply.

Still rather open since it does not say how many times you have to have sailed before.

It may be a matter of semantics.
Become a Member
Membership in the Seven Seas Society is automatic. All you have to do is sail. Full fare adult guests achieve Society Membership ON their second sailing, including all applicable benefits.

Since it says "on" rather than after, it was explained to me that when you book your 2nd cruise you receive the benefits that may apply to that cruise.