What do you see as the greatest challenge in implementing project-based learning?

As many have commented in Spiral Notebook -- Edutopia.org's blog -- launching a project-based-learning program is, to put it mildly, a complex undertaking. Implementing PBL means cultivating a rigorous, integrated curriculum that incorporates a wide spectrum of twenty-first-century skills, fully engaged instructors, and effective assessments. If you've developed or would like to develop project-based curricula at your school, what do you see as such a program's greatest challenge? We're interested in your opinion.