According to the Globe, the Romneys used a lawn-care company he connected with through his church. He evidently never asked about the legal status of those working for them. An early headache for a governor of a non-border state where immigration has been an issue (Romney got national news coverage of his objection to tuition cuts for illegal immigrants, for instance).
The company, by the way is called “Community Lawn Service with a Heart.” A Globe interview with one illegal who worked on the Romneys Belmont lawn revealed the details of an occasional “Buenos Dias” from the governor and the first lady of Massachusetts bringing workers water.
Asked about it at the RGA — seemingly hearing about the status problem for the first time — Romney replied, “Ah, geez.”