Hi Weijun,
This does appear to be a minor bug, but it is not in the data layer so it is not specific to SQL CE.
The origForumID is used only when moving a forum thread from one forum to another, in that case we need to keep track of the original forum id so that we can re-index it in the search index. So the consequence of this bug is possibly that if you moved a thread from one forum to another it would still show up as a search result in the original forum (in addition to the new one) since the vlaue was getting set incorrectly the original forum was not getting re-indexed.
This is a minor bug that has apprently been there a long time, but it is fixed now in the source code repository.
Thanks,
Joe