I just bought one but it is still in transit. I bought it a couple hours after Yosemite was announced and am really hoping that it comes with Mavericks. I had actually bought it before the release but had to cancel and reorder to change the shipping address because nobody was going to be around when it was due to arrive.
In 2011 I bought a MBP that came with Lion and I was able to go back to Snow Leopard as the hardware had originally been built for that. I would guess that you could back up to 10.9 on a mid-2014 MBP even if it came with 10.10.
This shows that they shipped originally with 10.9.4:
Mac OS X versions (builds) for computers
And have had no firmware updates:
EFI and SMC firmware updates for Intel-based Macs
Please Apple, I hope you shipped my machine with Mavericks so I don't have to use the abomination that is Yosemite.