What browser are you testing in?
What DOCType have you set?
How exactly are you declaring your CSS?
Are you sure you haven’t missed a ; before/after the overflow-y: scroll?
I’ve just tested the following in IE7 and Firefox and it works fine
<!-- Scroll bar present but disabled when less content -->
<div style="width: 200px; height: 100px; overflow-y: scroll;">
test
</div>
<!-- Scroll bar present and enabled when more contents -->
<div style="width: 200px; height: 100px; overflow-y: scroll;">
test<br />
test<br />
test<br />
test<br />
test<br />
test<br />
test<br />
test<br />
test<br />
test<br />
</div>