Samba 3.0.28-2+b1 running on debian sid cannot a share Fat32 directory with an iMac (leopard). When trying to write to the share from the iMac, permission is denied. Other (ext3) shares work normally. The linux PC dual boots Ubuntu, and when Ubuntu is running (samba 3.0.24-Ubuntu) the same Fat32 directory can be shared and written to from the iMac. Prior to samba 3.0.28-2+b1, debian sid also was able to share this directory normally. Write permission is denied even if chmod to 777. I believe this is a bug in samba 3.0.28-2+b1.
As of 2-18-2008. Previously, all smb.conf files on my home LAN used, "security = share" and were set up to be wide-open behind the router. My data isn't sensitive but I wouldn't want mischief in my system anyway. After changing the debian sid ws to "security = user", creating an smb user with a password, and restarting samba, I was able to write to the FAT shares. The other linux machines are still using "share" and can write to FAT just as before.
Closing this bug, "security=share" is really deprecated. And if it works fine using "security=user", then I think we should not invest effort into fixing it.