you might have to customise for this, i don't think any forum software's give you option to make forum thread your home page.
I am not sure if this would work but give it a try: Open your .htaccess file in root directory and add this line: DirectoryIndex filename.php?t=XXXXXX index.php index.htm index.html filename and Xs' are used as an example.. you will have to change it to your filename and topic id. Also, you forum needs to be in the home directory for the above to work. Another solution would be to mod rewrite the topic and then use the directoryindex directive.
Do you mean you want it as blahblah.com and not forum.blahblah.com? Yeah, that's possible. PC101 is set up like the former right now. Lyte