jruby-1.6.7

~/tmp % ruby -v
jruby 1.6.7 (ruby-1.8.7-p357) (2012-02-22 3e82bc8) (Java HotSpot(TM) 64-Bit Server VM 1.6.0_31) [darwin-x86_64-java]
~/tmp % ruby zz.rb
Decimal.new is a Decimal object?: false
Modulo op class is BigDecimal
Times op class is BigDecimal
Div op class is BigDecimal

rbx-1.9.3

~/tmp % ruby -v
rubinius 2.0.0dev (1.8.7 4f53f0ce yyyy-mm-dd JI) [x86_64-apple-darwin11.3.0]
~/tmp % ruby zz.rb
Decimal.new is a Decimal object?: true
Modulo op class is BigDecimal
Times op class is BigDecimal
Div op class is BigDecimal

ruby-1.9.3

~/tmp % ruby -v
ruby 1.9.3p194 (2012-04-20 revision 35410) [x86_64-darwin11.3.0]
~/tmp % ruby zz.rb
Decimal.new is a Decimal object?: true
Modulo op class is BigDecimal
Times op class is BigDecimal
Div op class is BigDecimal

test.rb

require "bigdecimal"

class Decimal < BigDecimal
end

puts "Decimal.new is a Decimal object?: #{Decimal.new("9876543210").is_a?(Decimal)}"

dec = Decimal.new("9876543210")
puts "Modulo op class is #{(dec % 2).class}"
puts "Times op class is #{(dec * 2).class}"
puts "Div op class is #{(dec / 2).class}"