{1=>[2]}となる事を期待したけど、この方法では駄目みたいですね。

% irb1.9.1
irb(main):001:0> h=Hash.new([])
=> {}
irb(main):002:0> h
=> {}
irb(main):003:0> h[1]
=> []
irb(main):004:0> h[3]
=> []
irb(main):005:0> h[1] << 2
=> [2]
irb(main):006:0> h
=> {}
irb(main):007:0> h[1]
=> [2]
irb(main):008:0> h[3]
=> [2]