I've been working as a programmer at my current company for about a year and a half. Recently my boss asked me into his office and asked me if I wanted to be the champion of a new set of features that we're adding to our web app. It would involve plenty of research, which I've been pretty good at in the past. He's offering me this as a challenge, something hard but possibly rewarding.
The problem is, I am planning to leave the company in a few months. My boss doesn't know this, and I don't want to reveal that I'm planning to leave until everything is lined up.
How can I turn down this challenge without telling him that I plan to leave?