What about a different approach to the difficulty prediction: Calculating how much of ASIC hardware is added to the network reach an equilibrium for the time to reach  ROI for the ASIC.

 My prediction is that the network will find an equilibrium where the time frame to reach ROI for an ASIC device is somewhere comparable to current GPU mining. With the 25 btc block award when ASICs hit the market, it can be argued that the ROI time frame will double from current GPU mining.
If current ROI predictions for a GPU has been about 6 months, a reasonable estimate would be that the network will reach an equilibrium difficulty with ASIC ROI of 1 year. (If ASIC ROI is over 1 year, less people are willing to invest in mining, thus reaching an equilibrium at 1 year) What is the difficulty with the current generation ASIC devices and prices?
If the difficulty will jump to a value where buying an asic would pay itself in 1 year, then it would be about 20x of the current value (assuming 60gh/s, 100w, price 1300 dollars, btc price at current level using bitcoinX.com calculator).
At the current ASIC prices of 50 Mh/dollar the network total of about 500 TH/s would cost about 10 million dollars. This is probably more than the price of the current GPU network.