Thanks to Chasbeen, I figured out how to make a true ring/donut in SVG. Note that the outer circle actually isn't closed, which is only apparent when you use a stroke. Very useful when you have many concentric rings, especially if they're interactive (say, with CSS hover commands).
For the draw command...
M cx, cy // Move to center of ring
m 0, -outerRadius // Move to top of ring
a outerRadius, outerRadius, 0, 1, 0, 1, 0 // Draw outer arc, but don't close it
Z // default fill-rule:even-odd will help create the empty innards
m 0 outerRadius-innerRadius // Move to top point of inner radius
a innerRadius, innerRadius, 0, 1, 1, -1, 0 // Draw inner arc, but don't close it
Z // Close the inner ring. Actually will still work without, but inner ring will have one unit missing in stroke
JSFiddle - Contains several rings and CSS to simulate interactivity. Note the downside that there's a single pixel missing at the starting point (at the top), which is only there if you add a stroke on.
Edit:
Found this SO answer (and better yet, this answer) which describes how to get the empty innards in general