There are several threads about this on bitcoin.org. You might want to read them. The value of bitcoins is determined by the supply-demand ratio. If more people sell bitcoins than buy them, the value goes down and vice versa. Halving the mining reward doesn't affect that. Also, this isn't some unexpected event. People have known about it since the beginning and have been able to plan accordingly, so there shouldn't be a drastic change in buy/sell behavior in the next few days. By this straight forward logic, the value of bitcoins shouldn't be affected by the halving of the mining reward. Of course, it can change for other reasons.