Russian planes were and are frequently intruding into Alaskan airspace. Palin was absolutely right on this, of course.
http://news.sky.com/skynews/Home/Sky-News-Archive/Article/20080641286622
http://www.cbsnews.com/blogs/2008/09/25/politics/fromtheroad/entry4479278.shtml
http://www.cbc.ca/canada/story/2009/02/27/arctic-russia.html