is just conjecture. A court of law may well decide that a Ripple IOU shall be an enforcable contract. That would be a distinct possibility today, and it will be more likely if people increasingly decide they want to use Ripple.
If you want certainty, write a contract to that effect, agreeing to rely on Ripple's impartial ledger as evidence. You may also want purchase orders and invoices. All this is business 101 and has absolutely nothing to do with Ripple itself.