With the latest Ruby in the 1.8 branch (including 1.8.5 preview 2), the colon meaning `then' seems to be obsoleted.
http://www.ruby-lang.org/cgi-bin/cvsweb.cgi/ruby/parse.y.diff?r1=1.307.2.36;r2=1.307.2.37
% /var/tmp/ruby/bin/ruby -v
ruby 1.8.5 (2006-07-23) [i686-linux]
% /var/tmp/ruby/bin/ruby -v script/server
ruby 1.8.5 (2006-07-23) [i686-linux]
./script/../config/../vendor/rails/activesupport/lib/active_support/inflector.rb:170: warning: colon will be obsoleted; use semicolon
./script/../config/../vendor/rails/activesupport/lib/active_support/inflector.rb:171: warning: colon will be obsoleted; use semicolon
./script/../config/../vendor/rails/activesupport/lib/active_support/inflector.rb:172: warning: colon will be obsoleted; use semicolon
./script/../config/../vendor/rails/activesupport/lib/active_support/json/encoders/core.rb:24: warning: colon will be obsoleted; use semicolon
./script/../config/../vendor/rails/activesupport/lib/active_support/json/encoders/core.rb:25: warning: colon will be obsoleted; use semicolon
./script/../config/../vendor/rails/activesupport/lib/active_support/json/encoders/core.rb:26: warning: colon will be obsoleted; use semicolon
./script/../config/../vendor/rails/activesupport/lib/active_support/json/encoders/core.rb:27: warning: colon will be obsoleted; use semicolon
./script/../config/../vendor/rails/activesupport/lib/active_support/json/encoders/core.rb:28: warning: colon will be obsoleted; use semicolon
./script/../config/../vendor/rails/activesupport/lib/active_support/json/encoders/core.rb:29: warning: colon will be obsoleted; use semicolon
./script/../config/../vendor/rails/activesupport/lib/active_support/json/encoders/core.rb:30: warning: colon will be obsoleted; use semicolon
./script/../config/../vendor/rails/activesupport/lib/active_support/json/encoders/core.rb:31: warning: colon will be obsoleted; use semicolon
=> Booting WEBrick...
./script/../config/../vendor/rails/activerecord/lib/active_record/base.rb:1900: warning: colon will be obsoleted; use semicolon
./script/../config/../vendor/rails/activerecord/lib/active_record/base.rb:1901: warning: colon will be obsoleted; use semicolon
./script/../config/../vendor/rails/activerecord/lib/active_record/base.rb:1902: warning: colon will be obsoleted; use semicolon
(snip)