Participants in this study will take imatinib mesylate by mouth every day for as long as they stay on study, which means as long as it works. Participants will also take R115777 twice a day for 2 weeks. This will be repeated every 3 weeks. The amount of each of these medications that participants take will depend on when they enter the study. The doses will be slowly increased from participant to participant until the highest dose that does not cause serious side effects is found.