The official Pokémon GTS website has put up an announcement stating it is closing down at 7:00 P.M. GMT on 14/09/10, just under a month from now. You can read the announcement here.

The closure is most likely due to the Japanese release of Pokémon Black/White which is known to have features which allow you to upload your Pokémon to the internet and interact with them there. This feature will most likely supercede the old Pokémon GTS website and so they are closing it down after 3 years of service.

This does not affect the ingame GTS! You will still be able to trade your Pokémon over the GTS. The change only affects the Pokémon GTS website.