testFont.js

function testFont(name) {
    name = name.replace(/['"<>]/g,'');

    var body  = document.body,
        test  = document.createElement('div'),
        installed = false,
        template =
            '<b style="display:inline !important; width:auto !important; font:normal 10px/1 \'X\',sans-serif !important">ii</b>'+
            '<b style="display:inline !important; width:auto !important; font:normal 10px/1 \'X\',monospace !important">ii</b>',
        ab;

    if (name) {
        test.innerHTML = template.replace(/X/g, name);

        test.style.cssText = 'position: absolute; visibility: hidden; display: block !important';

        body.insertBefore(test, body.firstChild);

        ab = test.getElementsByTagName('b');

        installed = ab[0].offsetWidth === ab[1].offsetWidth;

        body.removeChild(test);
    }

    return installed;
}

That's right, that's what seems to be happening, just on iOS 5.

EDIT: I'll see if I can make a reduced test case. It might possibly be something I'm doing in my CSS that messes it up.