According to a forum post by a Vodafone Community Manager the Gingerbread update for the Samsung Galaxy S has been suspended. By Google. There's no real detail on what caused the suspension of the update, but there is a clue in the forum post:
"As there are no known performance issues with the Vodafone variant of the device, we are working with Samsung to reinstate the roll out as soon as possible."
So a performance issue of some kind is the cause and Vodafone devices are unaffected. Updates usually always introduce a few new problems to take the place of the issues that they resolve so this must be a show stopper. A quick look around in all the usual places doesn't provide any user insight on what the issue or issues might be. The real question is how long the fix will take as Samsung is notoriously slow to update.