At the time of writing (2012-02-03 3:00 UTC), we are at block 165105. By block 210000, the block subsidy, or the portion of miner profits that is produced to mint bitcoins and serve as an extra bonus to miners, is scheduled to fall to 2500000000 of the smallest bitcoin units, or commonly 25 BTC. This gives less than 45000 blocks remaining before the subsidy falls. Although the technical effects are well-documented, the economic effects remain unknown. As we approach the date 2012-12-10, the current halving target if blocks continue being produced at 10 minutes every block, speculation should heat up. However, the consensus among Bitcoin speculators is that the effect of the reduction is already priced in.
Miners currently on average earn 0.05 BTC per block in transaction fees. This means that the current block subsidy, 50 BTC, makes up 99.9% of the total block reward. If no drastic changes to average transaction fees occur between now and the targeted date, this change will be a big blow to miners. Since many miners are very reluctant to turn down their rigs even when unprofitable, the difficulty is likely to only drop by a small amount as the total block reward nearly halves.
Luckily, even under a conservative economic appreciation model, transaction fees are likely to increase. While miners are happily chewing away on an average reward of 50.05 BTC per block now, the reward after halving may have taken a dip down to 25.10 BTC, still a drastic reduction.
One cannot ignore the difficulty decrease that is predicted as a result of the drop of subsidy, however it must be pointed out that even as the mining reward is only worth 20% of that in June 2011, the difficulty is 200% of that time. For this reason, mining is likely going to turn unprofitable shortly after the drop. Anticipating the drop, some dormant miners are likely to turn rigs on to complete their mining career for the time being, likely causing continued difficulty increases leading up to the change.
Pools, especially PPS ones and other pools which calculate reward based on 50 BTC, should begin preparing for this change. This means that PPS pools will drop their reward to half of what it is now in around ten months.
A 51% attack if difficulty does drop cannot be ruled out. Although unlikely, if the hashing power of the network does drop to a half of what it was before the change, it will be half as difficult for a 51% attack to be undertaken. Rapid growth in the Bitcoin economy before December, which is a possibity that cannot be ruled out, will also effectively prevent 51% attacks.
Bitcoin, in its current form, has not yet undergone such a severe change in block subsidy, and it never will undergo one of the same severity again.
Original post:
Unless something crazy happens, by now it is next to impossible having a block reward earlier than mid-November and later than late December. The current targeted time is 2012-12-09, but there is likely to be at least a small wiggle to a certain direction. If Bitcoin continues growth, the date may land in November, and otherwise could be as late as late December. With only 55970 blocks left until the block reward change, the difficulty will change only 26 more times before the block reward change. I am leaning to the side that the reward change has not yet been priced in, so prices of Bitcoin will increase considerably. Unfortunately, mining shares are likely to go down.