The answer seems to be yes. We’ve spotted a confirmation that the Nexus 5 will indeed feature a nano SIM card on Ukrainian carrier Life :) (yes, the market name includes the cute smiley!).
Now, that disclaimer would not have been there if the folks from Life :) did not know about the Nexus 5 sporting a nano SIM indeed. The Nexus 5 is not even on sale yet! So we take this as a confirmation of the support for nano SIM cards in it.
What’s even more interesting, though, is the fact that the carrier also mentions Android 4.4 KitKat in that list. Does this mean that the next Android version will mandate the nano SIM card standard for all phones? This seems unlikely, but that’s the only logical assumption after you read the disclaimer on the carrier’s website.
Looking back at Android devices, you’d notice that Google definitely has an affinity for nano SIM. Its Motorola handset unit has pushed nano SIM in the latest Motorola Droid series and the Moto X. This makes it all the more likely for the Nexus 5 to come with a nano SIM card.
source: Life :)