@sam
Already tried (as mentioned above) using forums as subdomain and also putting the htaccess details in a vhost.conf file. No joy either way.
@colin
“Quick and dirty” fix seems to do the trick.
Not actually sure if I’m going to go with bbpress anymore, I put a vanilla forum up in the meanwhile and it seems pretty decent. Although integration is nowhere near as slick with WordPress.
Did you get anywhere with this either oledole? I can’t figure it, and it doesn’t look like we’re going to get any help! 
I also have a bbpresss and wpmu installed at http://themotorcade.com and http://themotorcade.com/forums/ and that works fine, no 404 messages.
I tried using the htaccess settings from that site on my problem development site and no joy.
May be worth linking that thread over to this one (I don’t have a WP forum account just now), as it does affect both systems (working together).
If I turn off permalinks on bbpress it seems to work though, same with you?
I have used MultiViews as well as the more in depth htaccess and it didn’t change anything.
Here’s another thing, I tried instead to put the forums as a subdmain forum.mywebsite.com as opposed to mywebsite.com/forum and it still returns 404 errors. Odd.
Actually, it behaves exactly like: http://forums.hostmech.com/ in that situation.
The main URL is OK, but ANY other url shows as a 404.
Seems we’re not alone (my forum is not public yet so can’t post URL), but:
http://forums.hostmech.com/topic/3
Also hows as 404, but works in my browser (Firebug displays the 404, but we get there in the end and it shows the page).
http://www.seoconsultants.com/tools/headers/
Having the very same issue here. Main wordpress site is at / bbpress at /forum/ and although all appears fine the headers returned are 404 not found.
If anyone can shed any light on this weirdness that would be much appreciated.
I attempted moving the htaccess content into a vhost file on my server and put them in <Directory> sections, to see if that made any difference. No surprise really it didn’t.
I’m not an htaccess ot http expert and this seems just weird to me.
