If it is currently formatted as Mac OS Extended (Journaled) and has a GUID partition map you can non-destructively shrink the existing single partition and then create a partition in the free space and format it as MS-DOS(FAT) all in Disk Utility in Leopard.
Just drag the existing partition's bottom line upward some amount (it will not allow you to shrink it smaller than the existing data on it). Click the "+" sign at the bottom of the Partition window which will add a partition in the free space. Click on the new partition it creates to select it. Make sure that with that partition selected that you set the Format field under Volume Information to MS-DOS (FAT) and give it a name- then click the Apply button. If everything looks OK in the information/summary window that appears, then click OK. If not, cancel and click the Revert button at the bottom to try again.