You can run IIS on a physically different server to the SQL Server Web edition. In the end I got a support request in to Microsoft via the reseller that we purchased the software from. They first confirmed it, and we have installed it today and is running ok.
Once installed, we stepped through this blog post to make sure we had enabled remote connections: http://blogs.msdn.com/b/walzenbach/archive/2010/04/14/how-to-enable-remote-connections-in-sql-server-2008.aspx. In our web-edition, we only had to open the port on the firewall (in the evaluation edition, which is the full enterprise edition, we also had to enable TCP/IP).
Before we purchased the software, I asked this:
SQL Server uses "endpoints" to communicate with other applications/services. One of the endpoints is for HTTP access, but Web Edition does not support HTTP/SOAP endpoints! I'm not sure if this means our environment will work or not - there is another endpoint for TCP/IP which to my mind would mean it works, but I don't know if I need an HTTP endpoint or not?
We basically have 2 front end web-servers which host all our web-pages (most are still using classic-ASP and the Microsoft OLE DB provider to connect) ...
And received the reply:
The information Dan shares is correct, and also acknowledges the right way to access the server. It would be through TCP/IP endpoints and http endpoint not being available shouldn't affect the scenario.