The primary objective of this trial is to determine the proportion of patients with ER and/or PR positive, ErbB2 positive tamoxifen resistant metastatic breast cancer who achieve complete remission or partial remission or no significant change in lesion size for > 24 weeks from a combination of letrozole and trastuzumab. The study will also determine duration of response and median time to progression, evaluate toxicity, generate a tumor and serum bank, and analyze ErbB2 expression on circulating malignant cells during treatment.