Quick update: As announced by Martin Pitt on Thursday, Ubuntu 15.04 Vivid Vervet switched to booting with systemd by default today, replacing upstart.
This change affects Ubuntu desktop (and all Ubuntu desktop flavors) and cloud/autopkgtests (snappy was already using systemd) but not Ubuntu Touch.
According to Martin Pitt's announcement, Ubuntu Touch can't use systemd yet because most platforms currently run an ancient 3.4 kernel and "there's some porting work to do on the upstart jobs".
The plan is to use systemd for a few weeks and if "there are too many or too big regressions", Vivid will be reverted to boot with upstart by default.
In my quick test I didn't encounter any issues booting Ubuntu 15.04 Vivid Vervet with systemd but then again this was in a VirtualBox machine because I didn't have time to upgrade my laptop to Vivid yet. For a list of potential issues (and to report any bugs you may encounter), see the systemd Launchpad page.