I got this to work, but there's a few caveats I haven't resolved yet.
Anyway, this is how it's done:
Take your average input button:
<input type="submit" class="btn btn-success" value="Save">
Cut out the icon you want for your submit buttons from the glyphicons sprite file, make sure it's a 14x14 px image. Yes, in ideal circumstances you could reuse the sprite, and if anyone figures that out I'll be happy to hear how it's done. :-)
Once you did that, you can write css for your input button like this:
input[type='submit'] {
background-image: url('../images/submit-icon.png'), #62C462; /* fallback color if gradients are not supported */
background-image: url('../images/submit-icon.png'), -webkit-linear-gradient(top, #62C462, #51A351);
background-image: url('../images/submit-icon.png'), -moz-linear-gradient(top, #62C462, #51A351); /* For Fx 3.6 to Fx 15 */
background-image: url('../images/submit-icon.png'), -ms-linear-gradient(top, #62C462, #51A351); /* For IE 10 Platform Previews and Consumer Preview */
background-image: url('../images/submit-icon.png'), -o-linear-gradient(top, #62C462, #51A351); /* For Opera 11.1 to 12.0 */
background-image: url('../images/submit-icon.png'), linear-gradient(top, #62C462, #51A351); /* Standard syntax; must be the last statement */
background-repeat: no-repeat;
background-position: 5px 50%, 0cm 0cm;
padding-left: 25px;
text-shadow: 0 -1px 0 rgba(0, 0, 0, 0.25);
}
input[type='submit']:hover {
background-image: url('../images/submit-icon.png'), #51A351; /* fallback color if gradients are not supported */
background-image: url('../images/submit-icon.png'), -webkit-linear-gradient(top, #51A351, #51A351);
background-image: url('../images/submit-icon.png'), -moz-linear-gradient(top, #51A351, #51A351); /* For Fx 3.6 to Fx 15 */
background-image: url('../images/submit-icon.png'), -ms-linear-gradient(top, #51A351, #51A351); /* For IE 10 Platform Previews and Consumer Preview */
background-image: url('../images/submit-icon.png'), -o-linear-gradient(top, #51A351, #51A351); /* For Opera 11.1 to 12.0 */
background-image: url('../images/submit-icon.png'), linear-gradient(top, #51A351, #51A351); /* Standard syntax; must be the last statement */
background-position: 5px 50%, 0cm 0cm;
padding-left: 25px;
}
Works in Firefox 14, Chrome 21
Doesn't work in IE 9
tl;dr: With a bit of css you can automagically put icons on your submit buttons, but you need to put the icon in a separate file and it won't work in Internet Explorer.