Could you take a quick gander at this site please and help me understand why the white block has moved up into the header div please.

Only in IE/win :-)
Actually: the white block doesn’t really move up, but the header collapses because there’s only floats in it, so the white background on wrapper doesn’t get properly covered by the background on header.

Give IE/win a well-placed ‘hasLayout’ trigger…
#header {
height: 1%;
}

…and it will behave and paint backgrounds as intended.

Is there anyway I can show a gray semi-transparency over any elements with a certain class (”disabled”)?

.disabled {
background-color: #666;
width: 100%;
height: 100%;
-moz-opacity: 0.7;
opacity: 0.7;
filter: alpha(opacity=70);
}

The first rule provides opacity levels in Gecko based browsers (Camino, Firefox, SeaMonkey), the second to Safari (and Opera I think), and the third is an IE specific one, which I suspect is not valid code, but which works.

Font selection for the Web should be based on a generic font family to ensure an appropriate type style is rendered in any circumstances. Specific named fonts can then be declared in order of preference and tailored for specific platforms, but which to choose? This article compares the most common fonts for Windows, Mac and Unix family systems.