generating points from random to overdispersed on a raster grid.

Dear listers,

On a simulation purpose, I would like to generate data variying from 
random to various degrees of overdispersion on a raster grid.? For 
instance on a matrix 1000 x 1000, to select a number of pixels that 
could be considered spatially aggregated (with a possibility for tuning 
the degree of aggregation from "random" - no aggregation - to strong 
clusters).

Has anyone heard about how to proceed or an idea about it?

Best,

Patrick