It wouldn't make a difference if he does show it. "Don't ask/don't tell". If Romney's afraid to take a public stand on illegal immigration, then he'll also be afraid to implement a proposal that would significantly inconvenience illegals. Otherwise, how is it that illegals constantly qualify for Medicaid and other welfare benefits?
The indulgency that this country, and especially liberal states like Massachusetts, show towards illegals is very deeply entrenched in the political culture. A proposal like this isn't going to turn around and change all that through the back door.
Don't ask/don't tell will be a thing of the past after the next election cycle. Mitt's plan is to incrementalize the movement towards turning back the impact of the criminal alien element. Americans are sick of the inaction by GWB and congress up to now.