gistfile1.js

// This script exploits the fact that if you draw text on a 
// canvas with a font stack that contains no available fonts,
// nothing gets drawn at all. So create a font stack with a
// single web font, and we can determine when it has been loaded.
var webfontCallback = (function() {
  
  // Create a detached canvas with 1x1 dimensions
  var canvas = document.createElement("canvas"),
      context;

  canvas.width  = 1;
  canvas.height = 1;
  context = canvas.getContext("2d");

  // Setup context with your webfont at a huge size (1000px),
  // and a black fill style
  $.extend( context, {
    textAlign : 'center',
    textBaseline: 'middle',
    fillStyle : "#000000",
    font : 'normal 1000px "SomeWebFont"' // The web font you're loading
  });
  
  return function( callback ) {

    // Fill the canvas with black
    context.fillRect( 0, 0, 1, 1 )

    // Change fill style to white
    context.fillStyle = "#ffffff";

    // Render a huge 1000px X in the center of your canvas
    context.fillText( "X", 0, 0 );

    // See if the center-most pixel is still black
    if ( context.getImageData(0, 0, 1, 1).data[0] === 0 ) {
      
      // Reset fill style back to black and loop
      context.fillStyle = "#000000";

      setTimeout(function() {
        fire( callback );
      }, 0);

    } else {
      // Pixel isn't black, therefore the font has loaded,
      // fire the callback
      callback();
    }

  }
}());

webfontCallback( function() {
  console.log("SomeWebFont is loaded and ready to be used");
});

Just in case anyone else wonders:
Q: why 40px?
A: no reason really

Updated to use a 1x1 canvas, instead of 40x40, as per @csnover. =)

So, canvas elements can't be used before all web fonts has been loaded?

@peol - not quite. I added a comment to the top of the function. Basically, if you try and render text on a canvas with a font that is not available to the browser, nothing gets rendered at all, so we can test for when a font is available by checking the color of a pixel after attempting to render text.

@ralphholzmann OH, I totally missed the 'SomeWebFont' declaration... I blame sleepiness, woke up 15 minutes ago. :) Well done!