I'm considering upgrading my system and wondered whether I'd miss something if I choose a directx 10 card?
since when did developers had access to the directx 11 API?

Well if you do upgrade, you should definitely upgrade to a DX11 card so you don't have to upgrade again in the near future. I would think that the odds are pretty low that TESV will require it though. But pretty soon the number of titles that have DX11 options will start to increase and you'll be glad you got a DX11 compatible card.