Teams are restricted from trading away future first round draft picks in consecutive years. This is known as the "Ted Stepien Rule." Stepien owned the Cavs from 1980-83, and made a series of bad trades (such as the above-mentioned 1982 trade) that cost the Cavs several years' first round picks. As a result of Stepien's ineptitude, teams are now prevented from making trades which might leave them without a first round pick in consecutive future years.
The Stepien rule applies only to future first round picks. For example, if this is the 2005-06 season, then a team can trade its 2006 first round pick without regard to whether they had traded their 2005 pick, since their 2005 pick is no longer a future pick. But they can't trade away both their 2006 and 2007 picks, since both are future picks. Teams sometimes work around this rule by trading first round picks in alternate years.