SPDY is indeed always over SSL, but it requires TLS NPN (Next Protocol Negotiation), making it impossible to implement with older versions of OpenSSL or with any version of the Windows SChannel API.
This doesn't make it a particularly inviting protocol to use. I'd have to switch my entire SChannel layer for OpenSSL on Windows, and probably link statically to the version of OpenSSL with NPN on *nix.
If your servers support it, and a few major web browsers support it, then SPDY should work for most people. You can always fall back on HTTP, so if 80% of your users speak SPDY, that's a good situation.
Yep, the problem is that SChannel is part of Windows, so that it will take a new version of Windows to support it. Hopefully they are planning to implement support in Win8.
This doesn't make it a particularly inviting protocol to use. I'd have to switch my entire SChannel layer for OpenSSL on Windows, and probably link statically to the version of OpenSSL with NPN on *nix.