Samsung has just released a new firmware for the Galaxy S in India in the form of the I9000DDJG4 update. The new firmware is available via KIES, its PC application. The update brings bug fixes and a solution to the lag issue that the Galaxy S is plagued by once you have a number of applications open.
Early reports suggest that the performance of the Galaxy S is much better with I9000DDJG4 as compared to the firmware that it shipped with (DDJF3), however just a little lag remains. Rooting the Galaxy S is also possible post installing the new firmware. If you do not wish to update via KIES, then you can also grab the firmware from here to flash with Odin.
For more information you may want to follow this thread on xda developers.