You can book reservations for Planet Hollywood either way; the only difference being if you do it through Disney you can book it farther in advance. I wanted to make a reservation for Mother's Day (this was in early March), Planet Hollywood told me I would have to wait because they only take reservations one month in advance . As soon as I hung up the phone with Planet Hollywood I phoned Disney to make the reservation and didn't have any problem.
As of May, you could book thru WDW. However, they do not take any "same day" ressies. Same day is walk up only. They do accept walk up and there is usually little wait before 7 pm (according to the person at podium). We waited under 5 minutes!