Those who have already received a shipping confirmation should not be concerned by the message. For the rest of you out there, you will have to remain patient until we get more information from Google. And for the latter, this is a pretty big set back considering that the tech titan was trying to avoid a repeat of last year's launch of the Nexus 4 that saw global shortages of the device lead to unhappy customers who were turned away without being able to buy any variant of the phone.
This time around, it seems that LG might have had a better handle on the demand for the Nexus 5, and enough devices came rolling off the line. If Google's posting is to be believed, the problem is with order processing, not with the number of devices available. We still don't know how many units are involved, so the hope is that this is not a widespread problem.
source: Google via Phandroid