If you need something that will work on IE6 and IE7, check out the progid:DXImageTransform.Microsoft.gradient() filter. You can set both starting and ending colors to whatever you want the background to be, and use the alpha component of the color values to achieve transparency. And yeah, there's an actual opacity() filter as well... but the results look awful.
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Yes, but can you blame them for doing so if that's the only legal way they can hire programmers they want at the rate they can afford?
-- Nish on sketchy hiring practices