Hi,
there are a number of ways of doing semi transparent backgrounds, but there is no single standard way that is widely supported.
You can use a PNG image which is semi transparent as a background image, but this will not work in IE6 and older (but will for IE7, and all modern browsers)
You can use an rgba color for the background color - this is only a CSS3 feature, and not widely supported - though Firefox, Safari and Opera support this - afaik, IE7 does not.
You can also use the opacity property - this works on IE7 and all modern browsers - there's also a workaround for older IE's - to use an IE only filter.
The thing is, this makes the element and all its descendent semi transparent - not just the background - that means all the text, and graphics and so on.
So, there's no single perfect solution - but a combination of these should get you close - at least if you are ok giving users of IE6 and older a slightly less "incredible" experience than all other users
john