At the meeting at Bag End Balin says:
"You forget the front gate is sealed. There is no way into the mountain."
Is there some sort of magical protection on the door making it sealed or is it just Smaug has barricaded himself into Erebor sealing up the entrance?
POTENTIAL SPOILER IN ANSWER
From the Tolkien Gateway site:
Over time Smaug had broken and blocked all of the other gates to the Mountain (except for the secret Back Door), leaving only the Front Gate for his use.
Note that there is no mention of the front gate actually being sealed up, as I presume Smaug would have left it accessible for his own needs, so perhaps this was just a bit of hyperbole on Balin's part.
If you remember key points in both the first and second film, they say that Smaug has not been seen for sixty years:
"That dragon had slept for sixty years," - Elrond. "The Great Dragon Smaug has not been seen for sixty years," - Thorin.
In the second film, if you look carefully at when Smaug bursts out of the mountain, you'll see that he smashes his way through rocks and stone in order to get out. Quite easy for him, not so for dwarves. My guess is that before he went into the sixty year long sleep, he sealed himself in to better protect his horde of treasure from would-be thieves while he slept. It was easy enough for him to get out, but no one else could ever get in.
This is just my take on it. The books (which I haven't read) might be completely different.
Look close when he got out, you see the top of the mount where the dwarfs kept their archers is broken. Remember when he flies he sealed the front gate so no dwarf, elf or orc army could get in. In case anyone of them shows up he would just exit from the top of the mount that's high enough and kill them without touching the ground.
