I have a project file that every developer must change, but I don't want those local mods to ever be committed. How can I make svn commit ignore the file?

To do this, don't put that file under version control. Instead, put a template of the file under version control, something like "file.tmpl".

Then, after the initial svn checkout, have your users (or your build system) do a normal OS copy of the template to the proper filename, and have users customize the copy. The file is unversioned, so it will never be committed. And if you wish, you can add the file to its parent directory's svn:ignore property, so it doesn't show up as "?" in svn status.