Theoretically, the only limit to the number of messages per thread is the max table size of the database in use (i.e, MySQL). I've seen other phpBB forums with threads longer than ten pages long (phpbb.com's forums for example). As for editing the same post over and over, I don't think that's a problem either. It doesn't create a new record for each new post, just modifies the post currently in the database, so it's not really hurting anything.
Remember, we're leveraging the power of Open Source: Linux, MySQL (to a degree), Apache, PHP, and phpBB. All 5 have very competent programmers behind them, as well as thousands and thousands of eyes looking over the code every day making sure the software works, is stable (we aren't using beta versions), and is scalable. And all 5 run very well, even on "old" machines.
So don't worry about the software "exploding".
We'll most likely see a performance degrade before something explodes (phpBB is known to be pretty slow on very large forums), in which case we'll probably have already made provisions for it. Though, now that I've shot off my mouth, the server will probably explode tomorrow. Sorry, Dan.