I WAS rollerblading along the streets of Beijing a few weeks ago when I passed a poster of Christina Aguilera outside Workers'' Stadium. I was surprised to see it because I knew she wouldn''t be performing here. I skated up closer and saw that they were selling tickets for her Shanghai show. Since I''m a major Christina fan, I thought about it for just a few seconds, and responded to an impulse. I bought the ticket and last week took one of the new faster trains to Shanghai.