Chronic Lymphocytic Leukemia (CLL) and Small Lymphocytic Lymphoma (SLL) are similar diseases of the white blood cells and are typically treated the same way. Recent research shows that a key enzyme in CLL cells is responsible for cell survival. This enzyme is called LYN kinase. Laboratory studies show that inhibition of LYN kinase in CLL cells results in the death of CLL cells. Dasatinib has the ability to inhibit LYN kinase and, therefore, should have some effect on CLL cells. The purpose of this study is to see of the study drug dasatinib, in combination with fludarabine and rituximab, is safe and effective to use for people with relapsed or refractory CLL/SLL.