I've investigated how Opera handles the SVG inside <img> tag and I think they do this quite reasonably. If this would be helpful, here is a short summary (when refering to "foo.svg" I mean a "foo.svg" inside HTML img: <img src="foo.svg" />):
-First of all, as already decided about the eventual Firefox implementation - all scripts in "foo.svg" are ignored.
-In response to comment #56 - internal links in "foo.svg" are always ignored, whether <img> is surrounded by <a> or not.
-Animation - of course, scripted animation is not being executed, but the SMIL animation is, so if "foo.svg" contains declarative animation, then it's being animated (in my opinion this is something that web developers would be very glad to see once the bug #216462 is resolved).
-The <svg:image> tag - all images inside "foo.svg" are being rendered, whether this is "bar.png" or "bar.svg". But animations in "bar.svg" are ignored (even the declarative ones). So if "foo.svg" is included using HTML <img> then all internal images are treated as rasters (contrary to the situation when "foo.svg" is included using HTML <object> - then the "bar.svg" is animated, as expected).