For reasons unknown, a large number of players report that the game simply freezes up when trying to play after having downloaded the "Toy Box" pack. Some users report that clearing the system's cache and playing offline allows the game to load, but connecting to Live and downloading the game's latest update returns it to its non-functioning state. We've contacted Microsoft to find out what the problem is, and when we can expect a fix. You might want to consider keeping the "Toy Box" firmly closed for the time being.
[Thanks to everyone who sent this in!]