Quite often I use opacity to control how certain items look. For example, I want inactive items to be appear lighter to reflect the fact that they are inactive or not as important. To do this, I use <div style="filter:alpha(opacity=50); -moz-opacity:.50; opacity:.50;">blah blah blah</div>.
One of the things I've experienced is that bolded text does not always display correctly. To get around this, you can set the background-color to white and this will get around the display oddities. So, after changing my code to the following, the opaqued text looks great.
<div style="filter:alpha(opacity=50); -moz-opacity:.50; opacity:.50; background-color: white;">blah blah blah</div>