There are two:

Seattle-Tacoma International (SEA)
and
Wenatchee/Pangborn Field (EAT)
*Wenatchee is about 20 minutes from Leavenworth, WA

Most flights will take you to SeaTac, and then the option of either a small airline (e.g. Horizon Air or similar) or a Rental Car from Seattle (VERY scenic driving - we have never received any complaints from people who have made the drive) - both options will take you about the same amount of time (2 hours to Leavenworth).

If you (or anyone else who is reading this!) commit to coming to the conference, drop me a line; I may be able to help find a carpool from Seattle (there will be quite a few people coming over from that direction).

Everything in Leavenworth is within walking distance, and the stuff that isn't... lots of people will offer a ride.

~Mark