There is a bug in the version of Disk Utility that comes with Mac OS X 10.6.3, where if you format a volume as MS-DOS (FAT), it will not mount in Windows. This is caused by Disk Utility defaulting to give the MS-DOS volume a GUID partition table instead of a Master Boot Record partition map. The workaround is to partition the disk in Disk Utility, select one partition, select the MS-DOS (FAT) format, click on the Options button, and select Master Boot Record from the options. Apple is aware of this bug, and it will likely be fixed in their next major OS update.