I am lifetime Silver Wing (did never upgrade to Silver Wing PRO). I only used the program once (buying 4 USA Collection Zone Fares) since I joined in the late 90ies. Some changements of rules in the past made the program not very attractive to me anymore (having to start in the USA, removal of the 10% senior discount, etc.). But the initial signing in perks they offered some years ago were allready worth joining.

Reading the FAQs at the SilverWiong homepage (access only with SilverWing membership possible) it looks like the lifetime membership will go on (but many perks disappear):

What happens to my lifetime membership if I upgraded to Silver Wings Preferred?
Your Silver Wings Lifetime membership will revert back to active status when your Silver Wings Preferred membership expires on June 30, 2007.

As a Silver Wings Lifetime or Silver Wings Plus member, whom do I contact if I want to book travel after June 30, 2007?
To book a domestic or international Zone Fare ticket, please contact Silver Wings at 1-800-720-1765 and press option 2 for Travel. After June 30, 2007, the customer service number will be staffed Monday through Friday from 9 am to 5 pm (CST).

from the silverwing website (when I log-in as a lifelong member) it looks like Zone fares will be still available also in the future (but only to lifelong members):

2007 USA Collection Zone Fares (for travel starting April 1, 2007)

2007 International Zone Fares (for travel starting April 1, 2007)