RGBa is the new black, with support in most modern Safari 3, Firefox 3 and Opera 10, you can have semi transparent elements. Only there is no support currently in IE for RGBa.
However, you can easily fake it for backgrounds of block elements in IE with
filter:progid:DXImageTransform.Microsoft.Gradient(GradientType=1, StartColorStr='#aaRRBBGG', EndColorStr='#aaRRBBGG);
The filter gradient places a alpha channel colour gradient over the background. So as long the background is transparent, the start and end colours the same and the alpha (aa) channel a value greater than 0 (fully transparent) and FF (fully opaque), you get a block element in IE with a RGBa background. You can see an example of RGBa in action in IE …