This is some really cool stuff! I needed pretty much the same, but with horizontal gradient from white to transparent. And it is working just fine! Here ist my code:
.gradient{
/* webkit example */
background-image: -webkit-gradient(
linear, right top, left top, from(rgba(255, 255, 255, 1.0)),
to(rgba(255, 255, 255, 0))
);
/* mozilla example - FF3.6+ */
background-image: -moz-linear-gradient(
right center,
rgba(255, 255, 255, 1.0) 20%, rgba(255, 255, 255, 0) 95%
);
/* IE 5.5 - 7 */
filter: progid:DXImageTransform.Microsoft.gradient(
gradientType=1, startColor=0, endColorStr=#FFFFFF
);
/* IE8 uses -ms-filter for whatever reason... */
-ms-filter: progid:DXImageTransform.Microsoft.gradient(
gradientType=1, startColor=0, endColoStr=#FFFFFF
);
}