I have installed the latest 2.0 WP plugin, and there the feed shows a little different.
I found this code in the bbp-topic-functions.php:
(line 2994)
/bbpress/bbp-includes/bbp-topic-functions.php
<title><?php bloginfo_rss( ‘name’ ); ?> » <?php _e( ‘All Topics’, ‘bbpress’ ); ?></title>
If you alter that you could probably get the RSS title to say whatever you’d like.
I also found this:
/bbpress/bbp-includes/bbp-reply-functions.php
line 1372
<title><?php bloginfo_rss(‘name’); echo $title; ?></title>
I have ony 2 days of experience with bbPress, but this might help you…?
“If you alter that you could probably get the RSS title to say whatever you’d like.”
I can’t alter any files, because I am requesting this change for an existing WP.com support forum, for which I am the moderator ( but not admin!). And after requesting it elsewhere, they told me to come here in this forum and put my request in this forum …
BTW, I think you found just the title for the HTML page and not the rss feed.
Ok, I have no idea about the bbPress.org and wp.com relationshop.
However I believe that is the correct place to alter the RSS feed 
Here is the code in syntax:
// Display the feed
header( 'Content-Type: text/xml; charset=' . get_option( 'blog_charset' ), true );
header( 'Status: 200 OK' );
echo '<?xml version="1.0" encoding="' . get_option( 'blog_charset' ) . '"?' . '>'; ?>
<rss version="2.0"
xmlns:content="http://purl.org/rss/1.0/modules/content/"
xmlns:wfw="http://wellformedweb.org/CommentAPI/"
xmlns:dc="http://purl.org/dc/elements/1.1/"
xmlns:atom="http://www.w3.org/2005/Atom"
<?php do_action( 'bbp_feed' ); ?>
>
<channel>
<title><?php bloginfo_rss( 'name' ); ?> » <?php _e( 'All Topics', 'bbpress' ); ?></title>
<atom:link href="<?php self_link(); ?>" rel="self" type="application/rss+xml" />
<link><?php self_link(); ?></link>
<description><?php //?></description>
<pubDate><?php echo mysql2date( 'D, d M Y H:i:s O', '', false ); ?></pubDate>
<generator>https://bbpress.org/?v=<?php bbp_version(); ?></generator>
<language><?php echo get_option( 'rss_language' ); ?></language>
<?php do_action( 'bbp_feed_head' ); ?>
Oh, sorry! You are absolutely right. Without the context I thought this is not the right line, but with the context it is quite obvious that you are correct! Sorry again.
No problem 
But again, YOUR question was regarding the http://de.forums.wordpress.com
I am a fresh user of bbPress, but as far as I can see that forum us the “old” stand alone version of the bbPress. At least the code says so in the RSS generator:
<!-- generator="bbPress/1.1-alpha-2539" -->
`
In that case I am not the one that can help. I don’t even know who is leading the bbPress project 
Hope you get the help you need from others here!
Yes, I know. I wrote this is in my first post:
I see this on http://de.forums.wordpress.com and I know this is not the latest version running there.
Thank you for hoping with me …