[Sam Bauers]

We have bbPress physically located a subdirectory within WordPress.com (WPMU), you can call that “bbpress”. We do it this way because we point *.wordpress.com to the same host (well not exactly, but for the point of the example let’s say we do). Anyway, because on WordPress.com all blogs are given a subdomain, we need to intercept the forum subdomains and push them to the bbPress code before WPMU executes. That’s what this .htaccess code does.

You could do it without fewer rewrite rules if you didn’t use pretty permalinks.