Ok..i have just done this and am on Froyo now

....you can try it but its not my fault if it dont work!
First uninstall all lagfixes
Start up kies and connect your phone. Wait till kies recognizes the sgs.
Go to firmware upgrade.
If it doesn't report a JPM/JP6 upgrade close the upgrade window but do not close kies!
Go to windows start > run > and type "regedit"
Navigate to HKEY_CURRENT_USER/Software/Samsung/Kies/DeviceDB/
You will probably see a bunch of numbered keys (they look like folders) Find the key/folder that has strings that match your current firmware. So if you are on JPK look for that in the keys. If you have more than one galaxy S (it can happen) then match the imei with the phone you want to upgrade.
Change the following values so they look like this:
"SoftwareRevision"="I9000XXJF3/I9000SWC/I9000XXJF3/I9000XXJF3"
"ProductCode"="GT-I9000HKDXEE"
"HIDSWVER"="I9000XXJF3/I9000SWC/I9000XXJF3/I9000XXJF3"
Now go back to kies and click on firmware upgrade again. You should now see the JPM upgrade.
Other info:
First boot can take a while so let it sit for a couple minutes while you stare at that glowing S
Doing this will not change your product code.
3 button recovery mode still works.
This won't wipe your phone's apps, settings, messages or SD card. (at least not for me coming from jpk)
It doesn't matter what firmware you are on we are just using the JF3 value to fool kies.
You will lose root if you were rooted.
If kies crashed while extracting the firmware then try an older version of kies.
If your phone really hangs for more than 5 minutes at first boot then something probably went wrong. In that case start up in 3 button recovery mode and do a factory reset. Dont blame the registry hack, its kies that's the problem. As always flash at your own risk