Because the packaged OpenCV remains at the obsolete 2.4 release even still in 16.04 (Xenial Xerus), which is already in feature freeze, it is looking as though 2018 is the earliest that an Ubuntu LTS release will package OpenCV 3.x, which at that point will be already some 4 years old. (And the currently-shipped OpenCV 2.x branch will be going on 9 years.) See bugs #1516985 and #1540168.
This is not a great state of affairs for such an important package, but what exacerbates the situation is that OpenCV in Ubuntu (and in Debian) currently altogether lacks Python 3 support. Namely, the current OpenCV 2.4 packages support only Python 2.7, since Python 3 support was introduced only in OpenCV 3.0 [1], which was only packaged in Debian (experimental) last December.
This has already been a pain point for OpenCV users (and some packages requiring OpenCV) and is likely to become a burning problem over the course of this LTS release's lifecycle. The robotics community (e.g. ROS users), in particular, rely on Ubuntu LTS as a platform, and are going to be rather rudely surprised both by Xenial's old OpenCV release and still more by its lack of Python 3 support.
The relevant upstream Debian bugs are 799262 [2] and 792677 [3]. As mentioned, OpenCV 3.0 has already been packaged in Debian (packages libopencv-*3.0) and could be imported to Ubuntu. The Python bindings (python-opencv) still remain with a Python 2.7 requirement, however, which needs work upstream.
What can be done here? Would there be any prospect at all for a feature freeze exception so as to ship OpenCV 3 for Xenial? Once the base libraries are available, the Python 3 packaging is a much lesser problem and could even be done via PyPI.
[1] http://opencv.org/opencv-3-0-alpha.html
[2] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=799262
[3] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=792677
Status changed to 'Confirmed' because the bug affects multiple users.