It takes a small amount of logic and scripting within the interface to pull off.
For instance... if you are using phone number validation on a field called phoneNumber and you only want it required if a checkbox called callMe is checked.
You would use phone number validation and for the validated value you would browse to the phoneNumber field in the bindings, which would give you the value:
<?php echo((isset($_POST['phoneNumber'])?$_POST['phoneNumber']:"")); ?>
Now update it to use:
<?php echo((isset($_POST['callMe'])?$_POST['phoneNumber']:"(222) 222-2222")); ?>
This tells it to check to see if callMe was checked and if it is send the value of phoneNumber to be validated, otherwise it passes a valid phone number which would pass validation. The end result is exactly what you want.