Sorry, didn't check area51 for a while. Project has been re-written recently (again), this time its final version and already stable release candidate is available Also renamed from SimpleSVG to Iconify.
Yes, its cacheable. By default icons are cached in session storage. Also there is possibility to cache in local storage, but its disabled by default. If both are disabled for some reason, there is backup method of requesting icons that forms identical API requests, so it can be cached in browser. Also its possible to bundle commonly used icons with custom scripts, such as forum's main script.