Click to read a recommended LWN article about UIO
UIO is a framework that allows to implement drivers in userspace. This kind of thing causes much noise due to "monolithic vs microkernel" topic. To the surprise of many, the Linux ecosystem has actually supported userspace drivers for cases that had sense for a long time. libusb allows to access the USB bus from userspace and implement drivers there. This is why you don't have specific drivers for, f.e., your scanner or USB digital camera, programs like sane, gphoto, gnokii, gtkam, hplip, or even some music players like rhythmbox or amarok, use libusb to access the USB bus and talk to USB devices directly. The 2D X.org drivers that you configure in your x.org file are another popular example of drivers that not only they run in userspace, they also are portable to other unix operative systems (they're also an example of why userspace drivers can't avoid hanging your machine due to a bug in the driver that triggers a hardware hang). CUPS and programs accessing the serial port like pppd are yet another example of userspace programs accessing the devices directly - the kernel doesn't implement any specific LPT printer or serial modem driver, those userspace programs implement the driver that knows how to talk to the printer.
In other words, userspace drivers are not new. UIO is not a try to migrate all the Linux kernel drivers to userspace. In fact, a tiny (150 lines in the sample driver, including comments etc) kernel-side driver to handle some basic interrupt routine is needed as part of every UIO driver. UIO is just a simple way to create very simple, non-performance critical drivers, which has probably been merged more with a "merge-and-see-if-it-happens-something-interesting" attitude than anything else. For now UIO doesn't allow to create nothing but very very simple drivers: No DMA, no network and block drivers....
UIO Code: (URL...) UIO Documentation: (URL...) Sample kernel-side UIO Hilscher CIF card driver (URL...)