Soft Edges using CSS?

I am using RGBA to create a transparent background that overlays on top of an image. Works just fine. My questions is this: Is there a way to "soften" the edges of the box to where it flows more into the picture vs a hard edge.

Here is my CSS for the box:

#past{
    position:absolute;
    left:0;
    top:363px;
    background-color: rgba(34,34,34,0.6);
    /* For IE 5.5 - 7*/
    filter:progid:DXImageTransform.Microsoft.gradient(startColorstr=#99222222, endColorstr=#99222222);
    /* For IE 8*/
    -ms-filter:progid:DXImageTransform.Microsoft.gradient(startColorstr=#99222222, endColorstr=#99222222);
    z-index:10;
    padding:10px;
}

I know I can do this by creating a background image in Photoshop but I was looking for a CSS only way vs using an image.

Also I would prefer if at all possible for this to work in all browsers.

Thanks for the help. =>