Many of problems we've considered are challenging, but here are some that seem particularly difficult.
Find a way to get a contingent from PlanetMath to the International Congress of Mathematicians in Madrid this August. (Estimated cost $2000 per person.)
Explain free math to the broader mathematics community in a way they find not only acceptable but compelling.
Get a cool PlanetMath poster in every math department, community college, etc., in the USA (and/or the world).