Magical Index 2 Rumored for Fall
This isn't the most trusted source in the world for such news, but according to a Japanese Tsutaya store, a second season of Toaru Majutsu no Index / A Certain Magical Index will premiere this Fall. They had the announcement up on a poster, though no official word about a second season has come forth yet. So make sure to take this news with a grain of salt.
It's not as if a sequel series wasn't expected. The original light novel series is still going strong in Japan, & the end of the first season alluded to the fact that the animators still had more story to tell. The spin-off series "A Certain Scientific Railgun" was also a huge success, making it seemingly a sure thing for more stories from this world to become animated. The only thing that puts doubt into my head is that it seemed much more likely for JC Staff- the anime company behind the franchise- to do the third season of Shakugan no Shana first. & since season three has also been rumored to start this Fall, I doubt they'd be able to do both series simultaneously.
Source: http://www.animenation.net/blog/2010/05/20/index-2-this-fall