I recently installed an 11.04 alpha on my laptop. In the Additional Drivers program, it offers me "Experimental 3D support for NVIDIA cards". I went ahead and installed this to see how it works. Seems at the very least good enough. It runs Unity just fine.
What exactly is being installed with the experimental driver? How is it different from the default driver that Ubuntu uses when it detects an Nvidia card?
The driver it is talking about is the nouveau driver, which is FOSS. Typically that driver only does 2D. 3D support is not finished and is still experimental.
http://en.wikipedia.org/wiki/Nouveau_(graphics)
The idea is that this driver will eventually replace the other one, which is by NVidia and not FOSS, ie proprietary.
The most common case would be that this experimental driver is a pre-release version, in which case you are installing a driver that is newer than the released driver, but also still in development. Your driver may be more capable than the official non-experimental driver, but it may also give unwanted behavior. So, if you don't need the newest graphic drivers and are not willing to tolerate malfunctioning, you are probably better off with the non-experimental driver.