Initially the Captcha control was designed to look for the custom validated textbox in the NamingContainer of the Page (i.e. Page.FindControl(TextBoxID) ). The goal was to support scenarios when the RadCaptcha and custom validated textbox are in different NamingContainers (i.e. the RadCaptcha is in the NamingContainer of the Page and the TextBox is in the NamingContainer of a login control). This on the other hand introduced a "problem" when the TextBox is not in the Page's NamingContainer - the RadCaptcha cannot find the custom textbox and its UniqueID has to be supplied in order for the textbox to be found.
Because it is very unlikely that the Captcha control and the validated asp:TextBox to be in different NamingContainers, we have changed the code so that RadCaptcha searches the current NamingContainer (i.e. in the case with the login control - the NamingContainer of the login control). This way only the ID needs to be supplied and the TextBox will be found correctly. The fix has been in the latest internal build and you should download and test it in your project.
the Telerik team