There's one available on phpBB's site in the mod database but I believe that still has some bugs in it. There's topic for each mod that gets released, check the forum:

[phpbb.com...]

There's also quite a good one that does much more but is not affiliated with phpBB in anyway. A Google search will bring it up.

The dynamic URL's are not so much a problem as the duplicates. Although phpBB3 is much less of problem that phpBB2 it still has some produced through pagination and other features.

The only time the dynamic URL's really pose an issue is when you move a topic or if you do some large changes to the forum structure because the forum ID is included in the URL.

http://example.com/viewforum.php?f=69&t=100

Will become:

http://example.com/viewforum.php?f=70&t=100

If moved to the forum with a ID of 70. The forum ID is included for the online listing of where someone is at, you can safely remove the f=XX parameter AFAIK without any issues except for the functionality of the online listing.

A URL that just includes the topic parameter will work just as well.

phpBB is still, after many years of listening to complaints, heavily into adding session ID's to all its URI's and offering a "bot filter" system to remove them for bots. This causes problems with what you want too.

It would be better to move session ID's to cookies in my opinion, it's written as a good idea in G's WMT guide too, but that didn't get done before releasing 3.0 so...

Look for a hack to remove session ID's from the uri's (anonymous guests will lose posting ability but they shouldn't have it anyway) and then it's just a matter of getting the .htaccess file right.