My guess is still after Dec 15th but who knows
Anyway we can more or less be sure it is within a few weeks before Dec 9th and a few weeks after.
Update on that calculation since we are exactly nine months from the target date on the bet:
With block height of 170,390 reached, doing the math that gives us 275 days left, so still on target for December 9th as the day block 210,000 will be reached.
What is certain is that the number of bitcoins issued each day will be cut in half. Meaning if miners are generating $36K worth of bitcoins on December 8th, then the drop occurs, then starting on December 9th they will generate $18K worth of bitcoins -- though there would be some fluctuations in price that might cause those numbers to vary but essentially it will be a sudden 50%-ish drop in revenue.
We know today within a window of a couple weeks when that event will happen. We also know miners who pay for electricity won't mine at a loss for very long. We also know many miners have no other use for the GPUs.
A rational miner might consider selling off those GPUs before there is a glut on the market (though prices have dropped for used GPUs, prices currently are pretty decent -- compared to the discount other electronics see when being sold as used.) A rational miner might also look at the emergence of FPGAs in mining combined with this 50 BTC -> 25 BTC block reward drop, see the writing on the wall, and start taking action now.
Which would cause a rational speculator (if there are any) to suspect that difficulty will not continue rising and instead will start dropping again as the result of miners shutting down, and thus the date occurring later than December 9th.
But then we have history. When difficulty goes up, it tends to do so in bursts -- 8%, 15% or higher even. When it drops, it does so gradually. Which would mean even if we reach block 210,000 and the difficulty is no higher or lower than the level it is at today, we'll get there sooner than December 9th because the bursts had a bigger impact on calendar time than the drops did.
I still have no leaning as to which outcome is more likely. Tell me what the exchange rate will be and the number of FPGA units that will be sold over the next half year and the difficulty level at a future date becomes easier to predict.