Yes, work is proceeding on the SPDY Proxy proxy. The most visible public effort at the moment is adding support in Chromium for talking to a SPDY (aka HTTPS proxy). You can follow the progress in this issue in the Chromium bug system. On the server side, we have added support in the open source Apache module mod_spdy so that when used in conjunction with mod_proxy, Apache can act as a SPDY Proxy. In addition, I have implemented a stand-alone SPDY proxy based on the flip in-memory server code base, and just need to commit it to the Chromium tree.
To expand on this a bit - yes - efforts are underway. I'm not sure if that will be a public tool. One of these days, if nobody else gets to it, I'm going to hack on the in-mem-server to turn it into a proxy and put it in front of belshe.com :-)