Here's the go down.
Users who ran 1.5 and got the 2.1 OTA DID NOT get a bootloader update, so using the old bootloader, you can go to the root rom which runs .49 S-OFF (Engineering SPL)
Both the leaks and root roms have .49 BUT the root rom has older system information, so installing it from a leak rom which has newer system information, you will not be able to use it to root.
If your phone comes with a new bootloader, chances are, you cannot root.
There is a difference between the bootloaders:
2.1 Root ROM-> .49 S-OFF
2.1 Leaked Roms-> .49