<div id="scrollPane" style="height:150px;overflow-y:scroll;">
 <div class="inner">
  Terms of service jargon stuff here
 </div>
</div>
<input name="TERMS_ACCEPTED_YN" type="checkbox" id="TERMS_ACCEPTED_YN" value="Y"><label>I accept the blah, blah, blah.</label>

jQuery(document).ready(function() {
	jQuery("input#TERMS_ACCEPTED_YN").attr("disabled", true);
	
	var $box = $("#scrollPane"),
	$inner = $("> .inner", $box),
	innerOuterHeight = $inner.outerHeight();
	boxHeight = $box.height();
	boxOffsetTop = $box.offset().top;
	
	jQuery("#scrollPane").scroll(function() {
		if (Math.ceil(boxHeight - $inner.offset().top + boxOffsetTop) >= innerOuterHeight ) {
		jQuery("input#TERMS_ACCEPTED_YN").removeAttr("disabled");
	}
 });
});

<div id="scrollPane" style="height:150px;overflow-y:scroll;">
 <div class="inner">
  Terms of service jargon stuff here
 </div>
</div>

jQuery(document).ready(function() {
	jQuery("input#TERMS_ACCEPTED_YN").attr("disabled", true);
	
	var $box = $("#scrollPane"),
	$inner = $("> .inner", $box),
	innerOuterHeight = $inner.outerHeight();
	boxHeight = $box.height();
	boxOffsetTop = $box.offset().top;
	
	jQuery("#scrollPane").scroll(function() {
		if (Math.ceil(boxHeight - $inner.offset().top + boxOffsetTop) >= innerOuterHeight ) {
		jQuery("input#TERMS_ACCEPTED_YN").removeAttr("disabled");
	}
 });
});

<div id="scrollPane" style="height:150px;overflow-y:scroll;">
 <div class="inner">
  Terms of service jargon stuff here
 </div>
</div>

jQuery(document).ready(function() {
	jQuery("input#TERMS_ACCEPTED_YN").attr("disabled", true);
	
	var $box = $("#scrollPane"),
	$inner = $("> .inner", $box),
	innerOuterHeight = $inner.outerHeight();
	boxHeight = $box.height();
	boxOffsetTop = $box.offset().top;
	
	jQuery("#scrollPane").scroll(function() {
		if (Math.ceil(boxHeight - $inner.offset().top + boxOffsetTop) >= innerOuterHeight ) {
		jQuery("input#TERMS_ACCEPTED_YN").removeAttr("disabled");
		}
 });
});

This detects if a user has scrolled to the bottom of an overflow div and enables the checkbox. Can be used for terms of service acceptance, etc.

Div Bottom Scroll Detection

jQuery