Rather than pushing Mir into Ubuntu 14.04 as the first desktop release with Mir/XMir, they want to do it for 13.10, given that next year's release will be a Long-Term Support release where they must support it it on the desktop for a period of five years.
In Ubuntu 13.10 they will be using Mir by default in supported configurations (currently when using the open-source Intel/Radeon/Nouveau drivers) and will then be using XMir to run the Unity 7 desktop within the Mir Display Server. Unity 8 will be the first Ubuntu desktop natively handling Mir, but that's not baked yet. XMir is also needed for handling X11 application fallback support.
Oli Ries notes today, "Our Display Server Mir has gone from a proof of concept, sufficient to justify its announcement in March this year, to high quality, high performance component that we think will deliver the fastest, cleanest display experience for the Ubuntu platform. We are confident that all desktop environments and derivatives will work well throughout the transition, based on our ability to provide a full X compatibility layer."
With Ubuntu 13.10 for unsupported configurations, the X.Org Server will still be present. However, for Ubuntu 14.04 LTS the X.Org fallback session will be removed and will expect full Mir driver support. It's also not until Ubuntu 14.10 now where they want to run the next-generation Unity 8 shell.
The announcement about shipping Mir by default in Ubuntu 13.10 was made today on the ubuntu-devel list. Until it's all setup for the daily ISOs, Oli has written about the steps to run Mir on his blog. I'll be trying it out and attempting to do some benchmarks!