Code:
RewriteCond %{REQUEST_FILENAME} !-f [NC] 
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*).html$ index\.php?main_page=$1&%{QUERY_STRING} [L]
I modified the placement of my Google Analytics script earlier this afternoon & apparently haven't yet discovered the ramifications. Separate issue -- I'll take a look at that one now, but suffice to say the problem with ampersands pre-dates the Analytics tweak.