I've found this calculator:
http://coinplorer.com/Hardware/SimulateI find it more interesting, because most other calculators tend to either assume constant difficulty, or make simple linear projection (each month, N more GHash/sec that the month before).
The current progress seems rather exponential (with each passing months, a bigger quantity of ASIC hardware is shipped and put into production, and these machines are each more powerful than past models).
this will stop after about june 2014 as the generation of asics past 28nm will not nearly be a step as big as the step from GPU to FPGA and the step from FPGA to ASICs and not even the step from first generation ASICs to next generation ASICs.
Once 28/20nm ASICs are commonly used mining hardware, the difficulty will rise more gradually again.