What is the oxidation state of nitrogen in nitrate ion and what is its structure?
You are correct that the oxidation state is +5. The second period elements, particularly carbon, nitrogen and oxygen, do show variable oxidation states regularly.

The $\ce{NO3-}$ ion cannot be depicted using a single Lewis structure. However, its resonance structures are shown below:


What this means is that the actual structure of the nitrate ion is somewhere in between the three structures shown below. It is sometimes also depicted like this to show the symmetrical nature of the ion and the distribution of charges across it.



http://en.wikipedia.org/wiki/List_of_oxidation_states_of_the_elements
http://en.wikipedia.org/wiki/Nitrate#Structure