Hi Jan Hesselgren,
Unfortunately, this is a known issue, which will be fixed in the next version of RadControls (Q2 2008). Meanwhile, the only way to workaround this is to set the necessary property of the internal textbox control by using reflection.
You could add the code below in the constructor of the form, where the RadTextBox is:
object textbox = radTextBox1.TextBoxElement.TextBoxItem.HostedControl;
If you have any additional questions, please don't hesitate to write us again.
the Telerik team