Brian I'm in a similar position, with a backlog of six months or more. When I take an order I just tell them when I expect to have their knife done, and like Rich suggested, add a few weeks to cover for any unexpected scheduling problems. If they don't want to wait, there's really nothing I can do about that; nothing would make me put a later order in front of an earlier one just to satisfy some pushy customer (they've tried). Once in a while I do slip in something to satisfy myself, but that's really too rare.
The worst part of taking orders, for me, is that I never leave myself enough time to do those projects that I just want to do; almost everything I make is an order, and usually a repeat of something I've done before. That's the biggest drawback to taking orders, seldom being able to branch out to new patterns.
I've been telling people "No" for a while now, just because I want to make some of my own ideas, new knives that I haven't already done a dozen times. Saying No is hard, because that's reliable money that supports supplies and adds to my general cash flow a little (
very little!

) It's still going to be 2006 before I get caught up...
Some day I hope to get to the point where I just don't take orders at all; just make what I want and let the chips fall where they may...but the reality is probably somewhere in the middle.
Be happy you have that much demand for your work! You're on the right track.