Modders are already starting to chip away at GTA 5, but this is arguably the biggest breakthrough so far: OpenIV, a modding toolset used to access and edit Grand Theft Auto IV and Max Payne 3, has been updated to support GTA 5. Don't get too excited though, because thus far it doesn't allow the game's files to be edited, but it's probably only a matter of time.
What it does mean is that most of GTA 5's assets – including its textures – can be browsed, so we're probably not far away from discovering any well-hidden secrets kept inside the game.
It's worth mentioning that fiddling with GTA 5 in any way should be done with the utmost caution, and if you value access to GTA Online, you might want to refrain altogether: silly business may get you banned. Still, if it means we're one step closer to the kind of stuff we saw in GTA IV, then it'll be fun enough to watch from afar anyway.
In other news, someone has already made one of my dreams come true in the form of GTA 5 with Oculus Rift.