I have Windows Server 2008 server and IIS 7.0 being running on port 80 (main web server). Also I have Apache 2.2 running on port 8081 (for SVN).
Yes. This is called a "Reverse Proxy". See the following question for another solution (apparently it can be troublesome, but see Scott's answer):
You can also use ISAPI ReWrite for doing this, it seems to be a bit of a defacto standard for IIS, and uses a .htaccess compatible file format.