Y'know, a far simpler solution is to:
Create a CSS style:
Set your input field to that CSS style:
If you're doing this for a RadGrid autogenerated textbox field, ensure that you:
2a. add a GridTextBoxColumnEditor to your form;
2b. set it to use the allcaps CSS class you created in step 1 then;
2c. Set the RadGrid column's ColumnEditorID property in the RadGrid property builder to this GridTextBoxColumnEditor.
This will give your input text an ALL CAPS look as the user types, even if Caps Lock is not enabled on the keyboard.
3. Finally, update your input field to use its own value as uppercase:
TextBox1.Text = TextBox1.Text.ToUpper()
This will work even on RadGrid input fields. Just put the code in step #3 in the RadGrid.InsertCommand