Just today CodePlex released a bridge to allow subversion access to CodePlex. SvnBridge works by running locally on your box, and emulating a subversion server. The SvnBridge turns around and translates those calls into calls to the TFS server that CodePlex runs.
You can download SvnBridge from here. And while you are at it, check out SmartSVN. Its a free subversion client that has some really powerful tools that are easy to use.