This might be of some help: http://www.webmasterworld.com/forum83/200.htm
A relevant quote:
Most attempts to accomplish this were made by assigning the property and value: div{height:100%} - this alone will not work. The reason is that without a parent defined height, the div{height:100%;} has nothing to factor 100% percent of, and will default to a value of div{height:auto;} - auto is an "as needed value" which is governed by the actual content, so that the div{height:100%} will a=only extend as far as the content demands.
The solution to the problem is found by assigning a height value to the parent container, in this case, the body element. Writing your body stlye to include height 100% supplies the needed value.
html, body { margin:0; padding:0; height:100%; }