I'm trying to use mod_dav_svn with Apache on Win32 and I'm getting an error saying that the module cannot be found, yet the mod_dav_svn.so file is right there in \Apache\modules.

The error message in this case is a little misleading. Most likely Apache is unable to load one or more DLLs that mod_dav_svn.so relies on. If Apache is running as a service, it will not have the same PATH as a regular user. Make sure that libdb4*.dll, intl3_svn.dll, libeay32.dll and ssleay32.dll are present in either \Apache\bin or \Apache\modules. You can copy them from your Subversion installation directory if they are not there.

If this does not resolve the problem, you should use a tool like Dependency Walker on mod_dav_svn.so to see if there are any other unresolved dependencies.