Actually, the Ada Letters issues (beginning with March 2000 when I took over
as editor) are on-line but are password protected (ACM SIGAda members only).
See http://www.acm.org/sigada/ada_letters/issues/index.htm

As for copyrights, individual authors retain copyright on submissions to Ada
Letters, although they grant ACM permission to print and include in the
Digital Library.

--Martin