I’d suggest one tweak to crescentfresh’s answer – use
document.implementation.hasFeature("http://www.w3.org/TR/SVG11/feature#BasicStructure", "1.1")
rather than
document.implementation.hasFeature("http://www.w3.org/TR/SVG11/feature#Shape", "1.0")
to detect SVG. WebKit is currently very picky about reporting features, and returns false for feature#Shape despite having relatively solid SVG support. The feature#BasicStructure alternative is suggested in the comments to https://bugs.webkit.org/show_bug.cgi?id=17400 and gives me the answers I expected on Firefox/Opera/Safari/Chrome (true) and IE (false).
Note that the implementation.hasFeature approach will ignore support via plugins, so if you want to check for e.g. the Adobe SVG Viewer plugin for IE you’ll need to do that separately. I’d imagine the same is true for the RENESIS plugin, but haven’t checked.