This is the new version being used by wordpress...
RewriteEngine On
RewriteBase BASE/
#uploaded files
RewriteRule ^(.*/)?files/$ index.php [L]
RewriteCond %{REQUEST_URI} !.*wp-content/plugins.*
RewriteRule ^(.*/)?files/(.*) wp-includes/ms-files.php?file=$2 [L]
# add a trailing slash to /wp-admin
RewriteCond %{REQUEST_URI} ^.*/wp-admin$
RewriteRule ^(.+)$ $1/ [R=301,L]
RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule . - [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(wp-(content|admin|includes).*) $2 [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(.*\.php)$ $2 [L]
RewriteRule . index.php [L]
What would it look like in Nginx?
It depends how do you want to use WordPress Multisite.
For Multisite with Subdomain - http://rtcamp.com/tutorials/nginx-wordpress-multisite-subdomains-domain-mapping/
For Multisite with Subdirectory - http://rtcamp.com/tutorials/wordpress-nginx-multisite-subdirectories-nginx-map/
You will need to add some more rules depending on which cache plugin/mechanism you plan to use. (ref)