United already mostly eliminated 24 hour holds awhile ago (you can still technically get one by causing your credit card to be declined, be very convincing with a phone rep (no phone charges should apply), or using a GreaseMonkey script which tricks .bomb to trigger the hold.
24 hour refund policy is a regulation; US Airways recently implemented it due to it. UA holds however do not hold prices; if the fare bucket goes while the ticket is held, the PNR goes poof.