Yep, you can check in as early as you arrive. We checked in back in October, probably around 12:30pm. They actually had both of our rooms ready even. But that's just not something to expect. Odds are you can check in, and leave your bags with them.
Now even a few weeks ago we were able to check in at the DLH at 8 am! And they even had our room ready for us. Which was a complete shock. LOL We had a 8:30 am breakfast scheduled at PP, so we figured we'd just check in and have them hold our bags (we stayed there on our last night, after 5 nights at Hojo's). But it was really nice to have our room all set for us, and made it so we could go straight to the parks for the day after breakfast.