I was rudely surprised this week to find that the next Ubuntu LTS 
release currently in beta, Xenial Xerus (16.04), is--as it stands
right now--going to ship with OpenCV 2.4, not OpenCV 3.1 or even 3.0.
This notably also means that Xenial is going to ship with OpenCV
bindings only for Python 2.7, none whatsoever for Python 3.4+. This
despite Python 3 support being important to both Debian and Ubuntu's
ongoing release goals.
As Xenial is already in feature freeze , it is currently looking as
though April 2018 will be the earliest that an Ubuntu LTS release will
ship with OpenCV 3 and (hence) Python 3 support for OpenCV.
This is not a great state of affairs, given how many people rely on
Ubuntu LTS as their platform of choice, in robotics and other fields.
I've suggested a feature freeze exception for Ubuntu in order to ship
OpenCV 3 in Ubuntu 16.04 this coming month instead of two years from
now. I don't know if it's already too late to make that happen, but
asking beats waiting another two years...
If you would like to see something happen sooner rather than later,
please lend your support on Ubuntu bugs #1540168, #1516985, and
#1556156 [3,4,5]. If nothing else, mark the bug as affecting you, too
(you might need to create a Launchpad login if you don't already have
one). The relevant upstream Debian bugs are #792677 and #799262 [6,7].