I'm Randall Munroe, author of xkcd and creator of this project.
I live in Boston.
Thoughts: I wonder what percentage of Boston graticule is covered in poison ivy. It can't be a particularly large percentage, but it occurs to me that this project would get us perfect data on the matter. A truly random sample, as long as you go to every hash ...
I have been to geohashes in two states.
I have visited 5 geohashes, plus a few other alternate locations.
 Geohashes I've visited
2008-06-28, Boston Saturday meetup
2008-05-31, Boston Saturday meetup
2008-05-24, Boston Saturday meetup
 Visited during testing
Several others whose dates I actually lost, I'll dig them up later. All were before the site went up.