Lapatinib in Combination With Trastuzumab in Patients With HER2-Positive, Metastatic Breast Cancer

* Participants will be asked to undergo a biopsy of an area of the body where the cancer has spread. * Participants will be given a study medication-dosing calendar for each treatment cycle. Each treatment cycle lasts four weeks during which time you will be taking lapatinib, once per day. * Participants will receive Herceptin once every week or once every 3 weeks through a vein. * During all treatment cycles a physical exam will be performed and questions about the participants general health will be asked. Blood tests including chemistry and hematology will be performed to measure additional effect of the study drug and disease status. Photographs may be taken of the tumor to assess the response of the tumor to treatment. * CT scans will be repeated every 8 weeks to assess the effect of the study treatment on the cancer. Either a MUGA scan or echocardiogram will be performed 8 weeks and 16 weeks after the participant starts the study treatment. * Participants will remain on this research study for as long as they are benefiting from the study treatment.