I've been following the Gingerbread thread on Motorola's Droid X support forum to see if there's any new info for a little while now. Today, Matt, the forum manager had this to say in response to someone asking whether or not he'd be able to get the OTA if he had installed the leak:
"But it does give me an opportunity to say that you are right about the actual update. If you're not on the expected software, you won't get it. So this isn't a very good idea."
Going by that, I'd say you definitely would have to SBF back.
Because it's likely the actual GB download will be an improved version over what was leaked.
BTW - I saw that comment by Matt. It appears that he is referring to being on stock Froyo to get the official OTA update when it comes. But his response was to someone asking about installing a later version of GB - offered by a "private source" - than the one that was leaked. Thus, some confusion as to his meaning.
Since when the update is pushed to a device it verified version information and software before is allows the update to install I would say that since the last official build was 2.2.1 (2.3.340) and this is what the update will be looking for, you will have to SBF back to official build to install the update when it hits..
Along similar lines, what about ROMs? Running Apex 1.4.1, which is based on 2.2.1 (2.3.340). Do I need to SBF before pulling the OTA (if and) when it is available? If I am "running the expected software" when I pull the OTA, will theat fry my chances of getting it after I SBF?
It won't be long after the official update gets pushed OTA that someone will have the update.zip file up on a site to pull from a computer to your phone anyway, (similar to the way you got the leak on your phone,) so I don't know if SBFing back would be necessary.
it's possible that you still will have to, but these modders are getting dang good at manipulating the android system.