14 DAY TRIAL // Ubuntu 15.04 (Vivid Vervet) is scheduled to launch in April and just a few weeks of development are left, which means that Linux Kernel 3.19 is the most likely candidate for implementation in the distro.
Unlike Linux distros that have a rolling release model, like Arch for example, Ubuntu doesn't have the luxury of upgrading the Linux kernel after launch, except for some special circumstances. This means that the devs need to decide what version will be implemented in the system, and as it stands right now, that's probably the 3.19 branch.
The only Ubuntu operating systems that do get major Linux upgrades, from one branch to another, are the LTS releases. Ubuntu devs have numerous point releases (Ubuntu 14.04.1, for example) and they make the necessary upgrades and that often includes a newer Linux kernel.
Ubuntu 15.04 (Vivid Vervet) is coming soon
Linus Torvalds said just a few days ago that he was preparing to release Linux kernel 3.19, and the Ubuntu teams have been following this branch closely. They will adopt it in a couple of weeks, but it still needs a lot of testing. Given the fact that a regular development cycle for the Linux kernel is somewhere between 7 and 8 weeks, it's unlikely that Ubuntu 15.04 will get an even newer one.
"Our Vivid kernel has been rebased to the v3.18.4 upstream stable. It's been uploaded to the archive, 3.18.0-12.13. Please test and let us know your results. We will be rebasing to v3.18.5 shortly and uploading as well. We'll also be rebasing our unstable branch to v3.19-rc7 and will upload to our ckt PPA shortly," wrote Canonical's Joseph Salisbury on the official mailing list.
Ubuntu 15.04 has a few milestones coming up. One of them is the Feature Freeze, a point after which no more features are being added, and a Kernel Freeze, which is the same but for the Linux kernel.
You can download Ubuntu 15.04 Daily build right now from Softpedia and give it a spin. It's still under development, but it should be interesting to test.