bbPress 0.9.x and 1.x are A LOT different… No idea how you could get that work…
If you downloaded trunk via SVN, BackPress is included, else you’ve to download it from here.
Can you post a link to your forum so we can take a look at what’s happening?
You can download bbPress latest version, located here, via SubVersion.
Well, I believe this feature is missing mainly because bbPress is not a social network.
If you want to invite someone to a forum, just send him/her its web address! 
bbPress.org is running trunk (r2540 at this very moment), whom includes topic subscription. You’ve to download bbPress trunk to have that feature; I suggest getting it via SVN.
You just have to add bbPress CSS to WP header, I suppose.
You should ask this question in the BuddyPress forum.
The code above has some errors in it: $thisPage is used when calling bb_list_pages() recursively, but this param isn’t defined anywhere but in the function’s description… 
In function definition that param is instead named $parent_uri.
Moreover $kids <> "" should be replaced by !empty($kids) and I’m not sure that wordpress_mu_primary_blog_id has any meaning now that WP 3.0 has merged WP and WPMU…
Last, calling bb_list_pages() with no args, makes if ($depth == 0) true, resulting in an empty string returned. It would be good to use the same template of wp_list_pages(), which lists all pages if no param is passed to it (see docs). By the way, this could be the reason why you get nothing displayed when calling the function.
It’s a bit hard to explain…
I suppose you may try to download bbPress trunk and then BackPress trunk from here and extract the /includes folder into /bb-includes/backpress folder…
EDIT: you’ll need this too.
Note that the archive you can find on TRAC does not include BackPress: I suggest you to upgrade via SVN.
Did you download the archive from here or from the Download page on this site? The former is trunk, the latter 1.0.2…
Feature is available since [2471]. You need to have the trunk version of bbPress.
Are you using this plugin?
bbPress does not support it natively; I’d suggest Ajaxed Quote plugin. 
Oh, Avatar Upload is compatible up to bbPress 0.8.2.1, I’d suggest Bavatars.
EDIT: I’ve found a fix for the bug, I’m going to upload a new version in minutes. Please note that you must regenerate your .htaccess file in order for it to work!
Ciao! Can you tell me what plugins do you have active in your forum, please?
Note that if you didn’t download it via svn, you are probably missing BackPress…
1.1-alpha (trunk) has this option since [2431]
I’ve checked every BackPress version and none of them has $end at line 244…. weird…
The root of the problem is probably somewhere else.. no guess though..
What version of bbPress are you on? There is no $end at line 244 of that file, neither on the BackPress version shipped with bbPress 1.0.2 neither with trunk…
