It isn't corrosive, but if you don't clean your rifle, moisture can collect in the powder residue in the barrel (and other related parts, such as the gas system) and cause corrosion.  This will mostly apply to those living in high-humidity areas.

Corrosive ammo has salts in the primer compound, and the reason this causes corrosion is because the salts get deposited in the barrel and gas syttem (if applicable) and draw moisture, which rusts and pits the metal.  This is why you must clean parts exposed to corrosive ammo with fresh water (and a little mild soap) to remove the salts; you can then clean the barrel normally.

-Troy