I don't think they all need to support SPDY - as long as the proxy closest to the users supports it the stuff behind can run on regular HTTP (unless you're doing websockets stuff). nginx for example has a SPDY module, so if you put that in front of Jetty you can serve SPDY without any problems.

It also depends which SPDY features you want to take advantage of. SPDY has content push support, for example, so if you're serving a page and know which assets it will require, you can serve those without waiting for the client to receive your initial response and request them. So far as I know, the nginx SPDY proxy doesn't try to do any of this (and it probably doesn't make sense for it to do so).

If you're using ELB as the parent mentioned, SPDY wont be viable (yet).

I've wanted to add something like this to spdycheck.org. Right now I know of mod_spdy with Apache, and nginx has it too. Hosting providers are backing it in as well (Dreamhost has it as a option, with mod_pagespeed as well which is pretty cool). Hosted CMS solutions are another good place where this is happening. Wordpress.com provides it. I seem to remember hearing that Tomcat/Jetty has support, but I don't honestly know enough about how those frameworks plug into web servers to know if they can in fact at something like SPDY into the HTTP stack.

Anyone else have a list?