Updating to 2.2.1 and the new modem firmware (02.30.03) will render yellowsn0w useless. If you want to update, but keep your unlock, make a custom IPSW in PwnageTool that will not update the modem firmware. Everything that you needed to know before posting this was on the
Dev Teams Blog.
If you didn't mean the modem firmware update, just something else, I doubt that would be updated if you use PwnageTool, so your safe. The Dev Team would have said something by now if there was a problem.