If I am not mistaken, you will not see mod_rewrite in the info.php; I believe this is an Apache module, and Dreamhost servers are compiled with mod_rewrite supported.
The .htaccess file defines the re-write rules for Apache, not specifically PHP and is not entirely dependant on whether or not PHP runs as CGI or as an Apache module. If I have read the Knowledge base correctly, there are some PHP functions and environment variables that are not fully supported when PHP is run as CGI, but I have yet to experience a problem with mod_rewrite. Actually, I have not reserched it that much as, on the numerous mambo installations I have done on Dreamhost, it just works , running PHP as CGI.
My .htaccess operates with the entire RewriteBase variable section commented out. Following the logic that the comment said "if your server...", I tried it leaving it commented and it works fine. I suspect, however, that mambo is looking for a path from this variable, and per you description I would suspect that might be something like
I suggest leaving the "RewriteBase" stuff commented in the .htaccess and try it...if it doesn't work you can alway go back and plug in the "base" path.
You can tell if it iw working, by inspecting the urls produced when you hover a cursor over a link. "rewritten" urls will have "/" as seperators and not "?" in them when in the mambo "frontend".
I am sure a php pro around here can give you a more correct and technical explanation; i just answered because it worked for me and I wanted to help.