What I'm trying to accomplish is to "display:none" or "display:block" the following element whenever the <a> is onclicked.

Maybe this will help you to accomplish the flip flop part of your situation

<script type="text/javascript">
function flipflop(element){
var foo;
foo = document.getElementById(element);
if(foo.style.display != 'block')
{
foo.style.display = 'block';
}
else
{
foo.style.display = 'none';
}
}
</script>

now you can simply:
<a onclick="flipflop('SOME_ELEMENT');">Flip Flop some thing</a>

if this isn't what you need or if you can't apply the nextSibling thing on the above function, then please tell...

As you pointed out "document.getElementById()" returns not the id, but the href. (That really puzzles me and differs from the O'Reilly books on Javascript and Dynamic HTML).

Is this a known bug? If it is I will have to wrap my tags in such a way that I can find the other node relative to it some other way.