We want to visit Beijing and Shanghai around early April 2013. Is the weather good at that time?

Well, yes. You will get good weather to travel the two cities then. Beijing will hold an average temperature of about 13C or a little higher. While it is a little bit cold at night with the temperature of 8C. You can take thin sweater and light coat at that time.
As for Shanghai, it will warmer than Beijing, with the average temperature of about 19-20C. So you can take the thin coat. But sometimes, it may rain, thus the rainy coat or umbrella is needed.