Yes, some of it has been hacked, with limited success. The camera with the most hacks available is the D5100. You can remove the NEF compression, remove the time limit for video, allow JPEG Optimal Quality, and disable the third-party battery check, for instance.
Unlike the Canon cameras, which can execute modifications off of an SD card (external memory), Nikon cameras use an OS and chips that are restricted to execution in in-camera memory. That means that any "hack" has to fit in the firmware space and available internal memory to work. Generally, the higher end the camera is, the more precious that space is. Note that the D5100 options that have been uncovered so far are ones that are implemented in the D7000. The implication is that Nikon crippled the lower end camera intentionally, but uses mostly the same code base between cameras. Thus, there are opportunities for hacking, especially in the lower and midrange cameras.
But a few warnings are in order:
- You can "brick" your camera. A bad hack can render your camera inoperable.
- The hacking community isn't great at documenting. I've seen conflicting and incomplete instructions in the time I've been monitoring this development. Some descriptions are terse, some use technology terms you might not be familiar with, some assume knowledge you might not have.
- You're dependent upon others at the moment. The tools necessary to make any changes are homegrown. Vitaliy has been doing this for awhile now, originally for the Panasonic cameras (he was the source of the GH1/2 hacks), so he knows what he's doing. Still, you're not using tools developed by companies with which you may have some recourse should something fail.
- You're voiding your warranty. Nikon repair won't "un-brick" a camera should that happen to you and it's discovered you're using firmware hacks. They don't want the implied liability that comes with that.
That said, if you're still interested in what's going on, here are two sites to check out: