Serving multiple SSL websites on different domains on the same IP address requires the SNI extension and much fiddling around; it is ill-supported by old browsers. This is needed because with HTTPS, the header that tells the server which site to serve is still encrypted when the server needs to direct the request to the relevant application code. IPv6 will fix this by allowing you to easily give each application on your server its own IP as well as its own domain. This is simply because IPv6 creates a lot more addresses so we won't have to ration them like we do now.
There is no issue with IP addresses and SPDY perse. It's just that for SSL encryption (HTTPS) to work currently you would need your own IP address as older browsers (read IE < 8 on windows XP) don't support the SNI method of setting up HTTPS where you don't need your own IP address.
SPDY uses SSL and as it stands anything that doesn't support SNI will only accept one SSL certificate on one IP address. So if you run n certs on the same box you'd need n IP addresses.
