I agree with Jay that shipping during the Christmas holiday season can be slower than normal. I also agree that there are a lot of things that can cause problems for the host of a swap... which I think we discussed a bit way, way back when this idea was first being entertained. I think if I was hosting a swap I would set the deadline for about a week before I felt it necessary to receive the packages (although I wouldn't state that), and adjust the time for the seeds to be distributed accordingly. Or maybe it would be better to state that the packages must be
sent to the host by a certain date, rather than received. And requiring a tracking number could only be a good thing, IMO.
Packages can go astray -- I once had an order from Gardeners Supply delivered to a neighbor's house by mistake; the tracking said it had been delivered, but I clearly had not received it. Called our (very rural) PO, the postmaster talked to the mail carrier and she had delivered it to the house across the road. There are about 6 people living there, but they stuck the package in a corner (apparently without looking at the address label), thinking someone else had ordered it.
But what to do about packages not sent out on time? Not send that person any of the seeds they dibbed, and use the postage to return their late-arrival seeds? That doesn't really benefit anyone.... Maybe some sort of "registry" could be compiled of people who participate but don't follow the rules?
At any rate, I think there are a multitude of problems that can potentially arise -- one of the reasons I would not be eager to host a swap. Maybe it should just be left up to the individual hosts to determine how they will deal with things?