First of all you'll need to give .labelDiv an overflow:visible. Can you give a more detailed HTML example or a link to this page?
-edit- also the tooltip will need a width or at least min-width to make sure it doesn't adjust to the size of the containing div
Shouldn't you just adjust the top and left properties accordingly? What happens if you do?
EDIT
If your labels are set with, say 100px for simplicities sake, then adjusting the left to 100px will make the tooltip float over your input field. In this case there are two things to consider - you need to make sure your label div has overflow:visible and that it has a higher z-index than your input field.
If your label div is NOT set width you can adjust the right bound eg. right:0px which will put it on the right edge of the label. You can use negative numbers as well to make it break out of the label div in which case you will have to take the above two points into consideration as well.
We do need a little more info but try adding a z-index to the ".labelDiv" that is greater than the z-index of the input box. You may need to add a positioning to theinput box to make it accept z-index. I if it's a floated element I usually add "position:relative;float:left" to the element that I need lower but don't need to position it.
So my answer is z-index.
It should work.
EDIT
As faux paus as it might be. Would a negative right margin do the trick?
Couple of things, you should probably be using the <label> tag to declare the label for the input field. Using the title attribute on the input with the text of your tooltip will create the hover text you want over the input field and the browser will do all the work for you.
You guys were right, more HTML context was necessary. I ended up promoting the tooltip div all the way up to the body element, then manually positioning it relative to the appropriate label. I couldn't get the browser to do it for me without similar minor issues.