Yes. You can use rgba in both webkit and moz gradient declarations:
/* webkit example */
background-image: -webkit-gradient(
linear, left top, left bottom, from(rgba(50,50,50,0.8)),
to(rgba(80,80,80,0.2)), color-stop(.5,#333333)
);
(src)
/* mozilla example - FF3.6+ */
background-image: -moz-linear-gradient(
rgba(255, 255, 255, 0.7) 0%, rgba(255, 255, 255, 0) 95%
);
(src)
Apparently you can even do this in IE, using an odd "extended hex" syntax. The first pair (in the example 55) refers to the level of opacity:
/* approximately a 33% opacity on blue */
filter: progid:DXImageTransform.Microsoft.gradient(
startColorstr=#550000FF, endColorstr=#550000FF
);
/* IE8 uses -ms-filter for whatever reason... */
-ms-filter: progid:DXImageTransform.Microsoft.gradient(
startColorstr=#550000FF, endColorstr=#550000FF
);
(src)