Good points. I was thinking of just feeding/indexing the individual messages as records and then, for each result, linking to a page with the individual message (in the current Google Groups UI, you can get to this by clicking "show options" and then "Individual Message").

So the searcher wouldn't be confused by this scenario, but it does have the downside where searches on multiple terms present in different messages within the same thread won't return either message. The alternative of indexing the pages with the entire topic/thread conversation may be more difficult to manage though, because you'd want to essentially update (replace) each thread document with a new (slightly longer) document every time a new response comes in on that thread.

-Jeff