I'm one of the developers on mod_spdy. At this stage the module is reasonably stable and fully SPDY/2 compliant (an earlier poster incorrectly claimed that it does not support multiplexing. That is incorrect). That said, it is not as stable as the core Apache modules like mod_ssl. I consider it a "beta" module suitable for use in environments where you can tolerate some issues. There are currently web sites using mod_spdy successfully today.
We are actively working on making the module fully production ready and we plan to release DEB/RPM packages (in addition to supporting build from source) within a few months time.
We will announce the availability of packages and other updates on our discussion forum: https://groups.google.com/group/mod-spdy-discuss Please join the group if you'd like to stay up to date with mod_spdy news. Thanks!
PS: Steve mentions "For example, its implementation of the SPDY protocol is just an svn external reference that pulls in a chunk of the Chromium C++ source tree." and I want to clarify that this is absolutely the right thing for mod_spdy or any other SPDY-compliant C++ component to do. SPDY is still changing rapidly so by leveraging the core SPDY encode/decode logic from Chromium we can stay in sync and up to date as the SPDY protocol changes. IMO it would be a mistake to do it any other way.
RE: SSL you pay a small penalty for SSL, yes, but for all but the simplest web pages the performance benefits of SPDY will make up for the SSL overhead and give your users a secure connection as well.