As I have understood Steve's explanation on this topic:
Once the upgrade to vBulletin 4 is complete the existing Classifieds extension will no longer be compatible and there is no upgrade path for the existing Classifieds extension. There are several alternative extensions under consideration for handling Classifieds, but they will not be compatible with the old extension's listings.
As such, all the classifieds currently posted will likely disappear after the conversion to vBulletin 4, so the decision was made to stop any new Classifieds from being posted since the posts would only have a life of a few days.
Once the upgraded site has had a chance to stabilize a bit, a decision will have to be made as to which way to go with the new Classified extension alternatives and that alternative installed. Afterwards, everyone will likely need to repost their listings. As a result, it will likely be a couple weeks before a working Classifieds is implemented.
If I've got any of the details wrong (I'm new to this discussion and still catching up) those further in the know should feel free to correct me or expand on the above.
Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; Trident/5.0)