hello, i want to install ubuntu on my USB drive in persistent mode, however i would also like to be able to access my data from windows or mac or even another linux distro once install is complete, is this possible?

Yes, it's possible. You can install linux(including ubuntu) on fat or ntfs partition.
http://ubuntuwithme.blogspot.com/200...-live-usb.html for example
either you can install ubuntu itself on usb drive (without casper).

Persistent storage is not exactly the same as normal folder and files on a usb. You may be able to save files and or folders if you can mount the usb drive at the same time as you are booted to it. Not all distro's will let you do that. You may have to test as I don't use ubuntu on a flash. You will not be able to view the changes you made that are stored in the persistent or casper file.

FWIW, I have a Debian derivative called Knoppix installed on FAT formated flash, the persistent home directory is called "knoppix-data-img" and was in the Knoppix folder. I mounted it in Debian with command: mount -oloop knoppix-data-img /test, the test directory I created many moons ago and already existed, and I was able to access the data within. So this should not be an issue with Mac also, in Windows you would probably need third party software to mount and image.
Pendrive Linux has a quick and easy method, not sure if it formats FAT, but this is where I went to put together my Knoppix.

With regards to Windows, FAT/NTFS might not be strictly required as there is software for Windows that allows you to read ext2. Not sure about the MAC (no experience); might depend on the version that is in use.

I just had to know, so I found this application which worked like a charm on my Leopard. Click on the "view all files" link to the right of the green download link and select the .dmg version. Make sure to have the latest MacFuse installed as I had an older version from way back for writing to NTFS and it would not work till I installed the new one, rebooted.
There is another outfit that has software to do this but charge $$. If one has vmware-fusion, a linux guest will do the trick also.

all just sounds a little more complicated than what i wanted, perhaps i should just use web based storage e.g. dropbox