Works fine here:
Rgames> rasterize(matrix(1:6,3)/6,raster(matrix(1:8,4)/8),fun=mean)
class : RasterLayer
dimensions : 4, 2, 8 (nrow, ncol, ncell)
resolution : 0.5, 0.25 (x, y)
extent : 0, 1, 0, 1 (xmin, xmax, ymin, ymax)
coord. ref. : NA
data source : in memory
names : layer
values : 1, 3 (min, max)
Did you check to be sure you haven't accidentally removed mean
from your environment, or overwritten it w/ something else?