I did a lot of searching on this myself a while back, when I was building the $.supports() section of my bolster augmentation library. Here is what i settled on:
canSvg = !!(document.createElementNS && document.createElementNS('http://www.w3.org/2000/svg','svg').createSVGRect);
Tested on IE8+, works just fine.
EDIT
@SinaSadrzadeh makes an excellent point, and Modernizr is a fantastic resource. I recommend you use Modernizr for your project if you aren't already, but especially if SVG is not the only thing to test. If SVG is the only thing you are testing, bringing in an entire library is not necessary for that, but it doesn't allow for a lot of extensibility.