Putin's comment might be taken as it was portrayed – as an aside, or a little tidbit of information – if it weren't for the fact that Novorossiya has been brought up so often in recent days by pro-Russian activists, who have reportedly been chanting the word as they argued against staying with Kiev. Someone has even set up a Web site that appears devoted to bringing the historical region back.
If nothing else, Putin's comments are relatively accurate, historically: Novorossiya was won from the Ottoman Empire in the late 18th century. Its name, which means "New Russia," is a reflection of that. It became a part of the Ukrainian Soviet Socialist Republic in the early years of the Soviet Union, and remained a part of Ukraine after the collapse of communism.
Talking about Novorossiya fits in with Putin's broader habit of talking about a golden era of Russian empire, and using history to justify modern action. It's a similar action Putin made with Crimea, though in this case, the historical justification is a little harder to make: Crimea only became part of Ukraine in 1954, and you have to wonder what's to stop Turkey from deciding that its own claims on Novorossiya, earlier still than Russia's, are more valid?
In the modern world, things are more complicated. As the map below to the left shows, Crimea's large ethnic Russian population is not matched in any of the regions that once made up Novorossiya. However, as you can see from the map on the right, it makes up a big portion of the GDP of Ukraine.
It's not clear right now whether Putin has any real plan to annex Novorossiya, or whether it is just talk. But if Putin is hoping to foment chaos in Ukraine, it would appear that Crimea has shown him that history can be potent weapon.