There's an open-source SVG library in ObjectiveC, specifically for iOS (and OS X, although the OS X build is a bit behind):
https://github.com/SVGKit/SVGKit/
(I've been using this on iOS projects since the start of this year, it works well)
2018 update: OSX support is now in, and SVGKit renders most SVG's fast + accurately. On the downside, advanced text features aren't all supported (e.g. yes to gradient-fill, but no to complex text-layout).
As of early 2012, it's not a 100% implementation of the SVG spec, but there's a lot of people working on getting it there.