actually,PP's info is not correct: if it is a busy day (especially on weekends, even during slow times of year) they WILL check for your reservation, and if the parking lot at POLY is getting full they will turn you away if you are not a registered Poly hotel guest or not on the reservations print-out they have at the guard kiosk. We have eaten at Kona Cafe 3 times on weekend, and one time at 'ohana on a weekend...all 4 times, even though we had Disney hotel parking permit (for different Disney hotels) they still asked our names/and ID and also checked our name off on a list of reservations for the restaurants. One of those times there was a long line of cars backed up at the kiosk...we saw 3 cars actually turned away.
If you want to stay more than 3 hours, you can pay to valet park at the Polynesian...can leave your car as long as you like that way (no 3 hour limit when you pay to valet park...just have to pick car up before 6 AM next day, lol.)
You can also just park for free at the Transportation & Ticket Center...it's actually right next to the Polynesian Resort. So you can park there, take the tram to the drop-off area, then walk over to the Polynesian. About a 10 min. walk to the main building from the TTC