Is it OK/advisable to run both IIS6 and Apache on the same box in a live environment? Our developers want to use our live web server to install SVN using apache and I'm not to happy about it.
It's certainly possible; we do it on a couple of our dev boxes; as long as you can manage two sets of ports (if you don't have a separate interface for each service to bind to) and keep them both secure, there's no practical reason why not.
I'm more troubled by the fact that it looks like you're going to have a copy of your SVN repository on your production webserver when you finish, meaning that anyone who manages to compromise either IIS or Apache could potentially access it all.
In my experience, giving developers installation rights on your boxes is a bad idea - I've personally seen one of our servers left compromise-able by a badly-secured, out-of-date install of PHPMyAdmin that nobody told me about.
We do it and haven't run into any problems. Obviously they can't both use port 80 so you have to move one of them to a different port.
I'll add to what the others have said by saying that in a dev environment it's absolutely fine - just bind IIS and Apache to different ports. (In fact, the installer for Visual SVN Server doesn't use port 80 or 443 by default IIRC.) Just view them as two completely separate bits of software, just because they both happen to be HTTP servers doesn't mean there will be any kind of "conflict".
However like the others, I would not recommend putting source code on your live web server for security reasons (namely IIS or an app running on it being compromised)
I agree about not storing source code on a public live server, but nevertheless, if you want to run both servers on port 80:
When configuring both IIS and Apache to run on port 80 using different IP addresses, then see Setting metabase property DisableSocketPooling has no effect if your IIS 6.0 simply claims all available IP addresses. Adobe has some notes on this in How to configure Breeze and IIS to use port 80 on separate IP Addresses.
To have both IIS and Tomcat (the Java container; not Apache HTTP Server) available on port 80 on the same IP address (but using different domains, or different paths within the same domain), I've set up quite a few instances of Tomcat sitting behind IIS, using the ISAPI IIS-Tomcat Redirector. I can imagine something simular would be available for IIS and any other HTTP server. However, I doubt you want to do this to combine a production server with a SVN server.