By default Symbols are encoded as US-ASCII, 
Strings by default are encoded with UTF-8
Which leads to enforcing of US-ASCII when concatenation is performed (when Symbol is a first member of concatenation/interpolation expression).
$ ruby -v
ruby 1.9.3p0 (2011-10-30 revision 33570) [x86_64-darwin11.2.0]


# encoding: UTF-8
# coding: UTF-8
# -*- coding: UTF-8 -*-
# proof.rb
foo = :foo
bar = "#{foo}baz"
p bar.encoding

=> #<Encoding:US-ASCII>