Ruby, 137
----
    i=gets
    Hash[*%w[IV 4 IX 9 XL 40 XC 90 CD 400 CM 900 I 1 V 5 X 10
    L 50 C 100 D 500 M 1000].flatten].each{|k,v|i.gsub!(k,?++v)}
    puts eval i