In 4.1 (and earlier I assume), if you use code similar to the following (assuming ~/CMSFormControls/LargeTextArea.ascx is registered as cms:LargeTextArea):

<asp:Label runat="server" ID="lblForText" AssociatedControlID="txtSample" />
<cms:LargeTextArea ID="txtSample" runat="server" />

The rendered HTML will be the following:

<label for="txtSample" id="lblForText"></label>
<textarea name="txtRefundNote$txtArea" cols="20" 
id="txtSample_txtArea" class="LargeTextAreaTextBox"></textarea>
<input type="submit" name="txtSample$btnMore" value="..." 
onclick="ShowLargeTextArea('txtSample_txtArea');return false;WebForm_DoPostBackWithOptions(new WebForm_PostBackOptions("txtSample$btnMore", "", true, "", "", false, false))" 
id="txtSample_btnMore" class="LargeTextAreaButton" />

You'll note that the <label>'s for attribute doesn't work since there is no element rendered with that exact ID since the text box is now under a naming parent control.

Very minor bug of course, but a bug nonetheless.

Mufasa