Yes I think you're on the right track looking into Events. Something like this might work:

function OnGUI()
{
   if( Event.current.type == EventType.KeyDown)
   {
      spawn(Event.current.character);
   }
}

Your 'spawn' method would then be sent a character in the 'letter' argument variable.

However, then you need some way to tie your gameobject prefabs to the character code pressed. There are a number of ways to do this, none of them are particularly fun to implement.

One method would be to put your prefabs into the Resources folder. This allows you to reference them directly by name. If you put them in the Resources folder, and call them "Letter_a" through to "Letter_z", you could use code like this to instantiate them:

var prefab : GameObject = (Resources.Load("Letter_"+letter) as GameObject);
var newObject : GameObject = Instantiate(obj, position, rotation);

Hope this helps!
(untested, please report any errors)