When will FPGAs and ASICs 'replace' GPUs for mining? Please explain in detail, or provide an an estimate, at least.
Currently the hash rate is around 25 TH/sec.  It is expected that when the ASIC hit the market in Dec 2012, the new hash rate will be about 250 TH/sec.  The difficulty is directly proportional to the hash rate, so it should also increase 10x.  This would reduce the profitability of existing GPU miners by 10x.  Plus about the same time the block reward will be halved from 50 BTC to 25 BTC, again halving the profitability.  

The expectation is that once ASIC takes hold most of the GPU and FPGA mining will bleed out of the network.  You should expect to see an initial influx of ASIC and then a steady growth as mininers reach for their larger share of the market.