If you look at Romney's blow-outs last night, it was Massachusetts - where it was an open primary, a state with just over 11% registered GOP... the exit polls suggested only 45% of those voting were Republicans.

Some interesting numbers from MA actually...
total GOP registered voters: 466431
Total voters in GOP primary: 361392
Total GOP voters in GOP primary: 209894
GOP turnout in MA GOP primary: 45.000001%