My entry for RPCFN #4: http://rubylearning.com/blog/2009/11/26/rpcfn-rubyfun-4/

polynomial.rb

class Polynomial
  
  def initialize array
    
    if array.length < 2
      raise ArgumentError.new('Need at least 2 coefficients')
      exit
    end
    
    @result = ''
    @iterations = 1
    
    array.each do |item|      
      if item.abs > 0 
        @result << "+#{item}x^#{array.length - @iterations}"
      end
      @iterations += 1
    end
    
    @result.gsub!(/1x/, 'x')
    @result.gsub!(/x\^0/, '')
    @result.gsub!(/x\^1$|x\^1((\+|\-)\d)/, 'x\1')
    @result.gsub!(/\+\-/, '-')
    @result.gsub!(/\A\+/, '')
    
  end
  
  def to_s
    @result != '' ? @result : '0'
  end 
end

polynomial_test.rb

# assuming that your solution is polynomial.rb
require 'test/unit'
require 'test/unit/ui/console/testrunner'
require 'polynomial'
 
class Polynomial_Test < Test::Unit::TestCase
  def setup
    @p1 = Polynomial.new([-3,-4,1,0,6])
    @p2 = Polynomial.new([1,0,2])
    @p3 = Polynomial.new([-1,-2,3,0])
    @p4 = Polynomial.new([0,0,0])
    @p5 = Polynomial.new([2,0,-1,-6,0,0,0,3,12])
    @p6 = Polynomial.new([0, 0, 0, 0, 100])
    @p7 = Polynomial.new([1,0,0,0,0,0,0,0,0,0,0,0,0,0,1,0]) 
  end
 
  def test_first_negative
    assert_equal("-3x^4-4x^3+x^2+6", @p1.to_s)
  end
 
  def test_simple
    assert_equal("x^2+2", @p2.to_s)
  end
 
  def test_first_minus_one
    assert_equal("-x^3-2x^2+3x", @p3.to_s)
  end
 
  def test_all_zero
    assert_equal("0", @p4.to_s)
  end
 
  def test_a_longer_polynomial_with_an_initial_positive_two
    assert_equal("2x^8-x^6-6x^5+3x+12", @p5.to_s)
  end
  
  def test_one_hundred
    assert_equal("100", @p6.to_s)
  end
  
  def test_fifteen
    assert_equal("x^15+x", @p7.to_s)
  end
 
  def test_error
    e = assert_raise(ArgumentError) { Polynomial.new([1]) }
    assert_match(/Need at least 2 coefficients/, e.message)
  end
end
 
Test::Unit::UI::Console::TestRunner.run(Polynomial_Test)