URL field in Webforms
Well if you have tons of time you could implement a component type... but because that is unlikely you should just add a validation hook and a textfield to the webform. You could do a valid_url($form_state['values'][[your_field_name']); in your validation hook.

Someone asked this question on d.o previously here. You can find all sorts of examples on "things you may need to do special with a webform" here.

Another possible solution would be to install the Webform Validation module, add a regex validation to your field, and steal a known usable url validation regex like drupal has.