If you are trying to pre-authenticate your connection to the ISA server and then forward those credentials to the VisualSVN Server you are running into a limitation of NTLM authentication. A NTLM session is unable to be forward (a.k.a delegated) from ISA to another server because the NTLM authentication session is unique between the client and the ISA server. In other words, a NTLM authentication from client to ISA can't be proxied to the VisualSVN server. I would guess that Basic Authentication would work since that can be delegated to the VisualSVN server but you should not be doing this without SSL. Worst case is to use a self-signed ssl certificate.
If you must insist on using integrated credentials then you will need to make sure both your ISA server and your VisualSVN server are domain joined (in the same domain in fact) and then configure the ISA server to be allowed to delegate the (kerberos) credentials to the VisualSVN server. This is referred to as Kerberos Constrained Delegation (KCD). There are several articles out there on how to configure KCD that should help you but if you get into trouble I can provide additional info. My recommendation is to not use pre-authentication at the ISA level, just leave it to the VisualSVN process. Just configure the Publishing Rule so that ISA send the authentication request back to the client instead of handling it itself. Chris
