For those who use both Mac and PC. There is a way to run both PC and Mac portable apps on a USB flash drive seemlessly by creating a HFS+ formatted sparse disk image directly on the FAT formated USB flash drive.
First there is a website for portable apps for Mac at http://www.freesmug.org/portableapps where you should download Portable Firefox, Portable Adium, Portable Thunderbird, etc.
And of course http://www.portableapps.com for PC users.
Second, format your USB flash drive as FAT (MS-DOS) if it isn't already.
Third, on a Mac, go to "Disk Utility" and select your USB flash drive, then go to File-> New-> Blank Disk Image. Type a name for it, select a place for it (your USB flash drive), select a size, but make it around the size of your USB flash drive so you have room on the disk image for your portable Mac apps.
Then where it says "Format:" select "sparse disk image" and then click on "create."
After the sparse disk image you've created mounts, just drag and drop your portable Mac apps to it. When you're done with your apps, just unmount the disk image. When you are ready to use your portable Mac apps, just mount the disk image again when you are using a Mac. Easy.
Creating a sparse disk image means that you're creating a disk image that is not going to use up any free space on your USB flash drive until you add files to it.