In bitcoins case, mining will instantly become really unprofitable, will the relative scarcity drive up the price, will the network come to a screeching halt because miners turn off their rigs, halving difficulty
Most likely a combination of all of the above, exciting to see how Bitcoin and I0Coin will react to the first real change after the introduction of GPU mining
I anticipate network capacity will decline throughout 2012 in anticipation of the event. For whatever reason based on anecdotal evidence there is still a significant number of miners who mine at or below cost. It makes no sense. They aren't helping Bitcoin but they do it anyways out of some misplaced devotion to the network.
The combination of FPGA, looming block reward cut, and months of negative cashflow will finally IMHO cause them to pull the plug (or maybe sells rigs and buy reduced but more efficient hashing power in form of FPGAs).
It wouldn't surprise me in network capacity falls by 3TH in late 2012 even before the block reward cut.