When I include it in an <img> tag from a local XHTML file, Safari doesn't load it, either. Same problem in Firefox. It might be a bug in the "same domain" code.
However, if I strip out the preamble and include the svg element itself inline as part of an HTML page, it works for me in both Safari and iBooks (at least in OS X).
Similarly, if I use the <embed> tag instead of <img>, it works.
After you make that change, please file a bug at bugreport.apple.com. I would expect any SVG that works inline and passes validation to work correctly when referenced externally with an img tag, and I'm really not sure why this one doesn't.