Hi, Matt. As a buyer, when I order a custom knife, I mentally put that much of my savings (ok, my credit line

) aside for its purchase. I don't feel right about backing out of purchases, so you don't have to worry about collecting from me for the Poncho I'm in line for. If, say, someone ahead of me in line feels differently, I would be overjoyed to mail you payment at a moment's notice.
That being said, I think its just good business to contact the buyer to confirm their order anywhere from 1-3 weeks before you begin it, especially with longer waiting times. If a buyer orders a knife and gets an email a year later saying that it's been started, there's a good chance that, since a year has gone by, the buyer (1) may have let the exact date slip his mind, (2) may want to change or alter the final design in some small but significant way, or, at the very least, (3) may want a confirmation of the exact specs and details to make sure that he and the maker are still on the same page after all this time.
I agree with those who have said that the maker/buyer relationship is something special. I started buying custom knives for their quality, but also because a one-on-one transaction is a lot more meaningful to me than a purchase from a company. As a rough analogy, the bookcase that I made and the bookcase that my father spent weeks building me in his spare time have a value much beyond their function, while the one I bought from a store is just another piece of furniture to me. If someone is going to take the time to make something personally, I think a little more communciation between me and them can only be a good thing. It's not something that a maker needs to do, but it make the experience that much more personal for the buyer, which encourages repeat business and positive referrals.
Chris