I don't know about a forum but I do know Steiff has a 'community' for official Steiff Club members. Don't know where exactly you are living but this is the page with details on the Steiff Club membership worldwide: http://www.steiff.com/site/index.php?shopID=5000&L=4
This is for Steiff collectors living in any other country than Germany, Austria, Switzerland, UK, USA or France.
If you are living in one of those countries choose your country on http://www.steiff.com and then the button Steiff Club.
Hope this will help.
I turned up this forum with sections on different makes: http://www.teddybearsforum.com/forumdisplay.php?f=27
You have to register to use the site, but it might be interesting.
This is not really a forum, but a blog, written by Rebekah Kaufman, whose family have been collecting Steiff items for three generations.
Rebekah also writes for the Steiff magazine and other websites. I think most of the attention goes to vintage Steiff animals, but it's definitely worth checking out.
Peter & the bears
Teddybearsforum.com is my favorite bear forum. They have sections for all types of bears from artist bears to antiques. They even like to hear about my beloved manufactured bears. There's also teddybearfair.com. They have a forum, but they seem to be mostly geared to artists who want to show their bears. The section for collectors doesn't get a lot of traffic.