Mir may, or may not ship with Ubuntu 14.04:
Ubuntu 14.04 will be a long term support (LTS) release, so it remains to be seen if the Ubuntu developers will make the change to Mir with a LTS release of Ubuntu:
... it will be very interesting to see how the situation plays out for Ubuntu 14.04 since it's a Long-Term Support release. Modern Ubuntu LTS releases are maintained on the desktop for a period of five years so generally Canonical is conservative with pushing major user-facing changes into these versions.
But you will probably (maybe) have Mir with Ubuntu 14.10!