 DELAYED INTENSIFICATION PART 2: Patients receive imatinib mesylate PO QD on days 29-56, cyclophosphamide IV over 30-60 minutes on day 29, thioguanine PO on days 29-42, cytarabine IV over 1-30 minutes or SC on days 29-32 and 36-39, methotrexate IT on days 29 and 36, vincristine sulfate IV over 1 minute on days 43 and 50, and pegaspargase IV over 1-2 hours on day 43 in the absence of disease progression or unexpected toxicity. 