I was also wondering about that in this thread [webmasterworld.com] . Mainly because Google would be the ideal way to find old subjects/threads.
As far as I can see, there are about 235.000 messages (see the sum at the bottom of the forum index; [webmasterworld.com...] ).
I have never heard Brett post the total number of threads, but in this thread: [webmasterworld.com...] Brett mentions average number of messages and threads a day. Lets say 12 messages per thread as average over the years.
That would mean nearly 20.000 threads or 75% are missing at this moment in the Google index.
Lower Pagerank'ed sites have had deeper and fuller indexing, maybe the least internally crosslinked threads have been dropped? Or would Google decide to do less deep crawling on forums?
Then there is Freshness or the frequent spidering and short-indexing of a select number of pages:
Ever since Brett has added a shortlist of the latest participated threads on the member profiles, you can sometimes find Webmasterworld more often when searching in Google for topical subjects, as every once in a while Google decides to refresh-index moderators profiles. (the profile pages of moderators get a new link towards them with every new thread, as the moderator names always are on top of the thread and as far as I am concerned, recent new inbound links is a key criterium for freshness).
Try this for example Greetings from Inktomi [google.nl] . (this example will only last for a short while)
I guess the best remedy for fuller indexing will be Googeguy having a search experience trying to find some of his own threads back again ;)
Added: Then there is also ATW..28.813 listings within webmasterworld.com.. [alltheweb.com]