This is one of those 'damned if you do and damned if you don't' issues for the forum staff. Over the years, we've at times been quite liberal when it comes to thread drift, realizing that discussions that drift off topic will usually drift right back on topic if given time. But we've also received quite a bit of criticism for allowing it to happen. As a result, we've tended to modify our approach, and tolerate less tangential stuff in a technical or heavy help discussion than, say, chit chat in the Shade Tree area.
Most times, when a discussion has taken a different path and doesn't appear to be coming back on topic, staff will split off messages into a new topic with it's own subject line. It's unlikely to happen immediately, as staff will watch the flow of discussion and it rarely, if ever, happens as the result of a single message. Occasionally we'll drop a link in the original topic pointing to the new one, but most times we just move the stuff and nobody notices; Discussion continues in both the original and new topic.
We don't always get it right, and are unlikely to satisfy everyone, no matter how hard we try. In reality, it's always a judgement call, and different folks, including staffers, have a different threshold for what they consider drift or hijacking.
Oops, didn't mean to ramble on for so long. Hope nobody fell asleep.