Su Xda c'è scritto chiaramente che se hai la JM8 puoi procedere con il flash dy froyo, se invece arrivate da una versione DIVERSA dalla JM8 , dovete prima mettere la JM8.
Flash The New Firmware:
BEFORE anything, disable any LAGFIX.
FOR PEOPLE COMING FROM JM8 YOU JUST HAVE TO:
1. Extract the I9000XWJP6.rar file using password: samfirmware.com
2. Open Odin: select Pit 803 and put the only file you get (JP6_JP4_U_JPM.tar) in the PDA section of Odin.
3. Leave all other section blank
4. Don't thick "check repartitions" and Flash IT!
5. If the phone starts vibrating and giving force closes, just reboot in Recovery mode (VOL UP + HOME + POWER) and Wipe Cache/Data
INSTEAD, IF YOU COME FROM A DIFFERENT VERSION
First Flash I9000XWJM8 with RE-Partition and 512.pit. Then I9000XWJP6 following info above. (Thanks to SamFirmware)