[djbaxter]

1. That same information would probably be distilled from the content of the post/thread and a decent optimized page title without the need for mod_rewrite.

2. The majority of forum thread titles are so vague or generic or badly misspelled that the keyword-URL derived from them are virtually useless anyway.

Sample of recent threads on one large vBulletin forum:

Updated criterion
When members think they own the forum
Greetings from New England
Pricing Forums To Buy
What do you dislike the most?
Need a WordPress blog designer...
Best Free Software?
Editing members posts
Man's best friend indeed
Alert: Registerfly.com
Starting a forum help
How Many Members?
Question about cash mod and referral plus mod...
Board closed for upgrades - bad for SEO?
Movie/Game/TV Buffs required for New Site
1and1.co.uk?
Any major differences between Vbulletin?
Introducing {site name}
Should I Sell My Forum --

And another large webmaster forum:

No Link between Iraq and Al Qaeda
Webmaster's Blog
Sponser the SECOND nicest wordpress THEME ever SEEN
Google de ranked my directory
My grandma has just died
Warrior-type mascot
site wide exchange or link page exchange
CSS Site shows correct in Firefox but wrong in IE
Pr 3 directory - Free submission
Why
Owned Vbulletin License 4 Sale
Selling Links Cheap
Permanent PR6 links
PR4 Link exchange
[FREE] Need Made for Adsense (MFA) Script?
Any potential?
xbox forum 4 sell
The $48 Adsense website
More Forum Sign-Ups
Sponsor a cute Wordpress theme
Reviews my blog please
What do you think about Muslims and Jihad and Crusades

These are taken from "Today's Posts" on two vBulletin forums, edited only to remove thread titles that might identify the forum.

How many of those are going to convert to useful keyword URLs? Yes, a few perhaps... How many of those would be indexed for those same keywords from the content alone? How many others would yield better keywords from the content alone?