Hi,
> What if for example you want to ssi from a different domain name so
> the html is served from http://www.domain.com/ but the ssi uses
> www.other.com (which is also hosted on the same nginx instance).
>
>
You could set the SSI command to

<!--# include virtual="/domain2/oth-domain-url" -->

Then in your config have

location /domain2/ {
internal; # if you don't set this location to internal then your
web server could be used as a public proxy to that domain
proxy_pass http://domain2.com;
...
}

This would make a sub-request for http://domain2.com/oth-domain-url.

AFAIK you need to set the domain2 statically in your config file, i.e.
you can't use a regex in the domain name.

Marcus.