This is a multi-center, open-label, Phase I study of obatoclax administered every 2-week or weekly cycles, or as a Prolonged Infusion every 2 to 3 weeks to patients with Acute Myeloid Leukemia (AML), Myelodysplastic Syndrome (MDS), Chronic Myeloid Leukemia (CML)in Myeloid Blast Phase, Myelofibrosis, Previously-Treated Chronic Lymphocytic Leukemia (CLL), or Aplastic Anemia. Due to the PK/PD sampling schedule Cycle 1 will require overnight hospitalization. For the following cycles treatment may be administered on an outpatient basis but is at the discretion of the investigator. No investigational or commercial agents or therapies other than those described within the protocol may be administered with the intent to treat the patient's malignancy. Supportive care measures including those directed at controlling symptoms resulting from hematological malignancies are allowed.