gem install thread_backtrace

>> require 'thread_backtrace'
=> true
>> def a; b; end; def b; c; end; def c; sleep 10; end
=> nil
>> Thread.fork { a }.backtrace
=> ["(irb):2:in `c'", "(irb):2:in `b'", "(irb):2:in `a'", "(irb):3:in `irb_binding'", "(irb):3:in `fork'", "(irb):3:in `irb_binding'", "/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/irb/workspace.rb:52:in `irb_binding'", "/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/irb/workspace.rb:52"]