Unfortunately SPDY needs to be running on the web server where the TCP
connection from your users is terminated. In cases where you have an
proxy like nginx running in front of Apache, you would want to be
running SPDY in nginx. I believe there is support for SPDY in nginx
coming but i don't know the specifics.
You could also run nginx as a TCP proxy on port 443 to your Apache
instance and then run mod_spdy in Apache, but you would not get any
caching, etc benefits from nginx on port 443 in that case. There seems
to be an nginx TCP proxy module available:
https://github.com/yaoweibin/nginx_tcp_proxy_module but I am not at
all familiar with it.
Hope that helps!
-Bryan