Updating to .621 w/o installing rootkeeper results in no root.
No 100% guarantee that rootkeeper will work either, but better than nothing.
I saw in another forum where a CM9 user sbf'd due to an issue, grabbed the OTA thinking it was 605 and then lost root. Needless to say he's not happy.
IMO, the only resolution is to sell the X to someone who wants stock and buy one that has a custom rom on it OR use stock with the hope someone might fix it for another Moto phone and it happens to work with the X.