You don't need an exploit to install ROMs on Galaxy S variants (and in general most/all Samsung smartphones). The first stage bootloader supports partition rewrite over USB using a leaked "Odin" tool (or the open source "Heimdall" tool built through reverse engineering).
I used to use the Odin thing, but it makes me nervous. Heimdall doubly so. There's now that "ROM Manager" app in the market, which manages to flash firmware starting from userspace. I haven't looked into how it works, but I assume there's an exploit involved somewhere.
I believe ROM manager works after you have successfully gained root on your device and not before. It may have changed since I no longer have an android phone but when I had my droid the process was install a rooted version of your current OS then you upgrade from there. ROM Manager just made the installing other OS's and extras and keeping cyanogenmod up to date easier.
