I would hope for the best prepare for the worst but we were able to check in early on our last 2 trips. Jan '09 Poly and Nov '09 AKL. Both times we arrived just before 1 pm and were shown right to our rooms. Two things that may have helped were that we weren't there during peak season and we used an excellent travel agent who notified the resorts that we would arrive early with a toddler in tow.
That sounds like a pretty reasonable theory to me.I've heard that resorts have their largest amounts of check-ins between 1-3pm, so it's possible that the closer you get to this time frame, the more likely you'd have to wait until 3 for a room. Just guessing though